Pth1/Vam3p is the syntaxin homolog at the vacuolar membrane of Saccharomyces cerevisiae required for the delivery of vacuolar hydrolases

We constructed hybrid proteins containing a plant alpha-galactosidase fused to various C-terminal moieties of the hypoxic Srp1p; this allowed us to identify a cell wall-bound form of Srp1p. We showed that the last 30 amino acids of Srp1p, but not the last 16, contain sufficient information to signal glycosyl-phosphatidylinositol anchor attachment and subsequent cell wall anchorage. The cell wall-bound form was shown to be linked by means of a beta1,6-glucose-containing side-chain. Pmt1p enzyme is known as a protein-O-mannosyltransferase that initiates the O-glycosidic chains on proteins. We found that a pmt1 deletion mutant was highly sensitive to zymolyase and that in this strain the alpha-galactosidase-Srp1 fusion proteins, an alpha-galactosidase-Sed1 hybrid protein and an alpha-galactosidase-alpha-agglutinin hybrid protein were absent from both the membrane and the cell wall fractions. However, the plasma membrane protein Gas1p still receives its glycosyl-phosphatidylinositol anchor in pmt1 cells, and in this mutant strain an alpha-galactosidase-Cwp2 fusion protein was found linked to the cell wall but devoid of beta1,6-glucan side-chain, indicating an alternative mechanism of cell wall anchorage.     

Human syntaxin 7: a Pep12p/Vps6p homologue implicated in vesicle trafficking to lysosomes

INTRODUCTION AND AIMS: The authors report the results of an epidemiological study on breastfeeding from 1978 to 1993, followed by the same operators using the same method in the same territory, a semi-rural area of the province of Padua. In particular, they report the results for 1993 and compare them with those of earlier years and with those in Selvazzano, a town lying on the outskirts of the city of Padua. METHODS: A total of 963 of the 1115 women who had given birth during the study period were interviewed (85.6%). RESULTS: The frequency of breastfeeding rose from 61% in 1978 to 81% in 1993 and its mean duration from 4 to 6.3 months. The frequency of breastfeeding in Selvazzano was slightly lower (72%) as was the mean duration (5.8 months). The factor which most strongly influenced breastfeeding was the mother's work: its frequency was 66.1% for factory workers, 89.9% for housewives, 79.2% for office workers and teachers, 85.7% for shopkeepers-craftworkers and 75% for managerial staff. From the overall data it was observed that the frequency of breastfeeding increased in parallel to the length of time off work after birth: it rose from 69% among working women taking 3 months off work to 86% in housewives and those taking > or = 13 months off work. The level of education had less of an effect on the decision to breastfeed: the frequency of breastfeeding in mothers with high-school diplomas or university degrees was 85% compared to 79% in those with a lower level of education. According to this study the age of the mother did not influence the frequency of breastfeeding. CONCLUSIONS: The authors emphasise the need to carry out concrete new initiatives in favour of working mothers.     

Organelle membrane fusion: a novel function for the syntaxin homolog Ufe1p in ER membrane fusion

The fusion of endoplasmic reticulum (ER) membranes in yeast does not require Sec18p/NSF and Sec17p, two proteins needed for docking of vesicles with their target membrane. Instead, ER membranes require a NSF-related ATPase, Cdc48p. Since both vesicular and organelle fusion events use related ATPases, we investigated whether both fusion events are also SNARE mediated. We present evidence that the fusion of ER membranes requires Ufe1p, a t-SNARE that localizes to the ER, but no known v-SNAREs. We propose that the Ufe1 protein acts in the dual capacity of an organelle membrane fusion-associated SNARE by undergoing direct t-t-SNARE and Cdc48p interactions during organelle membrane fusion as well as a t-SNARE for vesicular traffic.     

Characterization of VPS41, a gene required for vacuolar trafficking and high-affinity iron transport in yeast

Mutations in the yeast gene VPS41 give rise to poor growth on low iron medium, severe alterations in vacuolar morphology, and cause the missorting of membranous and soluble vacuolar proteins. Our studies predict that VPS41 encodes a hydrophilic protein of 992 amino acids that contains no obvious signal sequence or hydrophobic domains. The deduced Vps41p sequence contains a domain rich in glutamic and aspartic residues, as well as a domain with resemblance to a region of clathrin heavy chain. We have also identified and sequenced putative VPS41 homologues from Caenorhabditis elegans, plants, and humans. The VPS41 homologues (but not the yeast VPS41 itself) contain a conserved cysteine-rich RING-H2 zinc finger at their COOH termini. Biochemical experiments suggest that VPS41 functions in post-Golgi protein processing: the deletion mutant exhibits defective high affinity transport due to impaired Fet3p activity and also exhibits defects in the processing and sorting of multiple vacuolar hydrolases.     

A yeast protein related to a mammalian Ras-binding protein, Vps9p, is required for localization of vacuolar proteins

In the yeast Saccharomyces cerevisiae, mutations in vacuolar protein sorting (VPS) genes result in secretion of proteins normally localized to the vacuole. Characterization of the VPS pathway has provided considerable insight into mechanisms of protein sorting and vesicle-mediated intracellular transport. We have cloned VPS9 by complementation of the vacuolar protein sorting defect of vps9 cells, characterized its gene product, and investigated its role in vacuolar protein sorting. Cells with a vps9 disruption exhibit severe vacuolar protein sorting defects and a temperature-sensitive growth defect at 38 degrees C. Electron microscopic examination of delta vps9 cells revealed the appearance of novel reticular membrane structures as well as an accumulation of 40- to 50-nm-diameter vesicles, suggesting that Vps9p may be required for the consumption of transport vesicles containing vacuolar protein precursors. A temperature-conditional allele of vps9 was constructed and used to investigate the function of Vps9p. Immediately upon shifting of temperature-conditional vps9 cells to the nonpermissive temperature, newly synthesized carboxypeptidase Y was secreted, indicating that Vps9p function is directly required in the VPS pathway. Antibodies raised against Vps9p immunoprecipitate a rare 52-kDa protein that fractionates with cytosolic proteins following cell lysis and centrifugation. Analysis of the VPS9 DNA sequence predicts that Vps9p is related to human proteins that bind Ras and negatively regulate Ras-mediated signaling. We term the related regions of Vps9p and these Ras-binding proteins a GTPase binding homology domain and suggest that it defines a family of proteins that bind monomeric GTPases. Vps9p may bind and serve as an effector of a rab GTPase, like Vps2lp, required for vacuolar protein sorting.     

Assembly of a ternary complex by the predicted minimal coiled-coil-forming domains of syntaxin, SNAP-25, and synaptobrevin. A circular dichroism study

The assembly of target (t-SNARE) and vesicle-associated SNAP receptor (v-SNARE) proteins is a critical step for the docking of synaptic vesicles to the plasma membrane. Syntaxin-1A, SNAP-25, and synaptobrevin-2 (also known as vesicle-associated membrane protein, or VAMP-2) bind to each other with high affinity, and their binding regions are predicted to form a trimeric coiled-coil. Here, we have designed three peptides, which correspond to sequences located in the syntaxin-1A H3 domain, the C-terminal domain of SNAP-25, and a conserved central domain of synaptobrevin-2, that exhibit a high propensity to form a minimal trimeric coiled-coil. The peptides were synthesized by solid phase methods, and their interactions were studied by CD spectroscopy. In aqueous solution, the peptides were unstructured and showed no interactions with each other. In contrast, upon the addition of moderate amounts of trifluoroethanol (30%), the peptides adopted an alpha-helical structure and displayed both homomeric and heteromeric interactions. The interactions observed in ternary mixtures induce a stabilization of peptide structure that is greater than that predicted from individual binary interactions, suggesting the formation of a higher order structure compatible with the assembly of a trimeric coiled-coil.     

The kinesin-related proteins, Kip2p and Kip3p, function differently in nuclear migration in yeast

The roles of two kinesin-related proteins, Kip2p and Kip3p, in microtubule function and nuclear migration were investigated. Deletion of either gene resulted in nuclear migration defects similar to those described for dynein and kar9 mutants. By indirect immunofluorescence, the cytoplasmic microtubules in kip2Delta were consistently short or absent throughout the cell cycle. In contrast, in kip3Delta strains, the cytoplasmic microtubules were significantly longer than wild type at telophase. Furthermore, in the kip3Delta cells with nuclear positioning defects, the cytoplasmic microtubules were misoriented and failed to extend into the bud. Localization studies found Kip2p exclusively on cytoplasmic microtubules throughout the cell cycle, whereas GFP-Kip3p localized to both spindle and cytoplasmic microtubules. Genetic analysis demonstrated that the kip2Delta kar9Delta double mutants were synthetically lethal, whereas kip3Delta kar9Delta double mutants were viable. Conversely, kip3Delta dhc1Delta double mutants were synthetically lethal, whereas kip2Delta dhc1Delta double mutants were viable. We suggest that the kinesin-related proteins, Kip2p and Kip3p, function in nuclear migration and that they do so by different mechanisms. We propose that Kip2p stabilizes microtubules and is required as part of the dynein-mediated pathway in nuclear migration. Furthermore, we propose that Kip3p functions, in part, by depolymerizing microtubules and is required for the Kar9p-dependent orientation of the cytoplasmic microtubules.     

Identification and characterization of a human protein kinase related to budding yeast Cdc7p

The Cdc7p protein kinase is essential for the G1/S transition and initiation of DNA replication during the cell division cycle in Saccharomyces cerevisiae. Cdc7p appears to be an evolutionarily conserved protein, since a homolog Hsk1 has been isolated from Schizosaccharomyces pombe. Here, we report the isolation of a human cDNA, HsCdc7, whose product is closely related in sequence to Cdc7p and Hsk1. The HsCdc7 cDNA encodes a protein of 574 amino acids with predicted size of 64 kDa. HsCdc7 contains the conserved subdomains common to all protein-serine/threonine kinases and three "kinase inserts" that are characteristic of Cdc7p and Hsk1. Immune complexes of HsCdc7 from cell lysates were able to phosphorylate histone H1 in vitro. Indirect immunofluorescence staining demonstrated that HsCdc7 protein was predominantly localized in the nucleus. Although the expression levels of HsCdc7 appeared to be constant throughout the cell cycle, the protein kinase activity of HsCdc7 increased during S phase of the cell cycle at approximately the same time as that of Cdk2. These results, together with the functions of Cdc7p in yeast, suggest that HsCdc7 may phosphorylate critical substrate(s) that regulate the G1/S phase transition and/or DNA replication in mammalian cells.     

The yeast histidine protein kinase, Sln1p, mediates phosphotransfer to two response regulators, Ssk1p and Skn7p

The Saccharomyces cerevisiae Sln1 protein is a 'two-component' regulator involved in osmotolerance. Two-component regulators are a family of signal-transduction molecules with histidine kinase activity common in prokaryotes and recently identified in eukaryotes. Phosphorylation of Sln1p inhibits the HOG1 MAP kinase osmosensing pathway via a phosphorelay mechanism including Ypd1p and the response regulator, Ssk1p. SLN1 also activates an MCM1-dependent reporter gene, P-lacZ, but this function is independent of Ssk1p. We present genetic and biochemical evidence that Skn7p is the response regulator for this alternative Sln1p signaling pathway. Thus, the yeast Sln1 phosphorelay is actually more complex than appreciated previously; the Sln1 kinase and Ypd1 phosphorelay intermediate regulate the activity of two distinct response regulators, Ssk1p and Skn7p. The established role of Skn7p in oxidative stress is independent of the conserved receiver domain aspartate, D427. In contrast, we show that Sln1p activation of Skn7p requires phosphorylation of D427. The expression of TRX2, previously shown to exhibit Skn7p-dependent oxidative-stress activation, is also regulated by the SLN1 phosphorelay functions of Skn7p. The identification of genes responsive to both classes of Skn7p function suggests a central role for Skn7p and the SLN1-SKN7 pathway in integrating and coordinating cellular response to various types of environmental stress.     

Functional analysis of Rrp7p, an essential yeast protein involved in pre-rRNA processing and ribosome assembly

During the functional analysis of open reading frames (ORFs) identified during the sequencing of chromosome III of Saccharomyces cerevisiae, the previously uncharacterized ORF YCL031C (now designated RRP7) was deleted. RRP7 is essential for cell viability, and a conditional null allele was therefore constructed, by placing its expression under the control of a regulated GAL promoter. Genetic depletion of Rrp7p inhibited the pre-rRNA processing steps that lead to the production of the 20S pre-rRNA, resulting in reduced synthesis of the 18S rRNA and a reduced ratio of 40S to 60S ribosomal subunits. A screen for multicopy suppressors of the lethality of the GAL::rrp7 allele isolated the two genes encoding a previously unidentified ribosomal protein (r-protein) that is highly homologous to the rat r-protein S27. When present in multiple copies, either gene can suppress the lethality of an RRP7 deletion mutation and can partially restore the ribosomal subunit ratio in Rrp7p-depleted cells. Deletion of both r-protein genes is lethal; deletion of either single gene has an effect on pre-rRNA processing similar to that of Rrp7p depletion. We believe that Rrp7p is required for correct assembly of rpS27 into the preribosomal particle, with the inhibition of pre-rRNA processing appearing as a consequence of this defect.     

Aip3p/Bud6p, a yeast actin-interacting protein that is involved in morphogenesis and the selection of bipolar budding sites

A search for Saccharomyces cerevisiae proteins that interact with actin in the two-hybrid system and a screen for mutants that affect the bipolar budding pattern identified the same gene, AIP3/BUD6. This gene is not essential for mitotic growth but is necessary for normal morphogenesis. MATa/alpha daughter cells lacking Aip3p place their first buds normally at their distal poles but choose random sites for budding in subsequent cell cycles. This suggests that actin and associated proteins are involved in placing the bipolar positional marker at the division site but not at the distal tip of the daughter cell. In addition, although aip3 mutant cells are not obviously defective in the initial polarization of the cytoskeleton at the time of bud emergence, they appear to lose cytoskeletal polarity as the bud enlarges, resulting in the formation of cells that are larger and rounder than normal. aip3 mutant cells also show inefficient nuclear migration and nuclear division, defects in the organization of the secretory system, and abnormal septation, all defects that presumably reflect the involvement of Aip3p in the organization and/or function of the actin cytoskeleton. The sequence of Aip3p is novel but contains a predicted coiled-coil domain near its C terminus that may mediate the observed homo-oligomerization of the protein. Aip3p shows a distinctive localization pattern that correlates well with its likely sites of action: it appears at the presumptive bud site prior to bud emergence, remains near the tips of small bund, and forms a ring (or pair of rings) in the mother-bud neck that is detectable early in the cell cycle but becomes more prominent prior to cytokinesis. Surprisingly, the localization of Aip3p does not appear to require either polarized actin or the septin proteins of the neck filaments.     

Depletion of a Drosophila homolog of yeast Sup35p disrupts spindle assembly, chromosome segregation, and cytokinesis during male meiosis

This paper addresses the effects of availability and anchoring-and-adjustment on people's beliefs and values concerning environmental issues. The first three studies focus on lay people's perceptions of the causes of large scale environmental risks, the second series of three studies deals with how people value environmental goods and how much they are prepared to pay to mitigate environmental risks. In studies 1-3 we investigate the effects of availability and anchoring-and-adjustment on estimating the contribution of various factors to large scale environmental risks. Highly complex risks such as acid rain and global warming tend to be associated with multiple causes, and our results show that estimating the role of these causes is clearly affected by availability and anchoring-and-adjustment. Both have sizeable effects and persist over time. Moreover, corrective procedures only seem to have a limited effect. Availability and anchoring-and-adjustment not only play a role in judging the possible causes of risks; they also play a role in research attempting to assess the public's willingness to pay (WTP) to protect our environment. The outcomes of WTP surveys are often used as a tool to help policy decision making. In the second part of this article we present three studies on this issue. Results provide further evidence of the impact of the two heuristics on the outcomes of WTP research. Implications for research and practice are briefly discussed.     

Ndj1p, a meiotic telomere protein required for normal chromosome synapsis and segregation in yeast

The Saccharomyces cerevisiae gene NDJ1 (nondisjunction) encodes a protein that accumulates at telomeres during meiotic prophase. Deletion of NDJ1 (ndj1Delta) caused nondisjunction, impaired distributive segregation of linear chromosomes, and disordered the distribution of telomeric Rap1p, but it did not affect distributive segregation of circular plasmids. Induction of meiotic recombination and the extent of crossing-over were largely normal in ndj1Delta cells, but formation of axial elements and synapsis were delayed. Thus, Ndj1p may stabilize homologous DNA interactions at telomeres, and possibly at other sites, and it is required for a telomere activity in distributive segregation.     

Identification of a penicillin-binding protein 3 homolog, PBP3x, in Pseudomonas aeruginosa: gene cloning and growth phase-dependent expression

A homolog of Pseudomonas aeruginosa penicillin-binding protein 3 (PBP3), named PBP3x in this study, was identified by using degenerate primers based on conserved amino acid motifs in the high-molecular-weight PBPs. Analysis of the translated sequence of the pbpC gene encoding this PBP3x revealed that 41 and 48% of its amino acids were identical to those of Escherichia coli and P. aeruginosa PBP3s, respectively. The downstream sequence of pbpC encoded convergently transcribed homologs of the E. coli soxR gene and the Mycobacterium bovis adh gene. The pbpC gene product was expressed from the T7 promoter in E. coli and was exported to the cytoplasmic membrane of E. coli cells and could bind [3H] penicillin. By using a broad-host-range vector, pUCP27, the pbpC gene was expressed in P. aeruginosa PAO4089. [3H]penicillin-binding competition assays indicated that the pbpC gene product had lower affinities for several PBP3-targeted beta-lactam antibiotics than P. aeruginosa PBP3 did, and overexpression of the pbpC gene product had no effect on the susceptibility to the PBP3-targeted antibiotics tested. By gene replacement, a PBP3x-defective interposon mutant (strain HC132) was obtained and confirmed by Southern blot analysis. Inactivation of PBP3x caused no changes in the cell morphology or growth rate of exponentially growing cells, suggesting that pbpC was not required for cell viability under normal laboratory growth conditions. However, the upstream sequence of pbpC contained a potential sigma(s) recognition site, and pbpC gene expression appeared to be growth rate regulated. [3H]penicillin-binding assays indicated that PBP3 was mainly produced during exponential growth whereas PBP3x was produced in the stationary phase of growth.     

Hus1p, a conserved fission yeast checkpoint protein, interacts with Rad1p and is phosphorylated in response to DNA damage

The hus1+ gene is one of six fission yeast genes, termed the checkpoint rad genes, which are essential for both the S-M and DNA damage checkpoints. Classical genetics suggests that these genes are required for activation of the PI-3 kinase-related (PIK-R) protein, Rad3p. Using a dominant negative allele of hus1+, we have demonstrated a genetic interaction between hus1+ and another checkpoint rad gene, rad1+. Hus1p and Rad1p form a stable complex in wild-type fission yeast, and the formation of this complex is dependent on a third checkpoint rad gene, rad9+, suggesting that these three proteins may exist in a discrete complex in the absence of checkpoint activation. Hus1p is phosphorylated in response to DNA damage, and this requires rad3+ and each of the other checkpoint rad genes. Although there is no gene related to hus1+ in the Saccharomyces cerevisiae genome, we have identified closely related mouse and human genes, suggesting that aspects of the checkpoint control mechanism are conserved between fission yeast and higher eukaryotes.     

Efficient initiation of S-phase in yeast requires Cdc40p, a protein involved in pre-mRNA splicing

The S. cerevisiae CDC40 gene was originally identified as a cell-division-specific gene that is essential only at elevated temperatures. Cells carrying mutations in this gene arrest with a large bud and a single nucleus with duplicated DNA content. Cdc40p is also required for spindle establishment or maintenance. Sequence analysis reveals that CDC40 is identical to PRP17, a gene involved in pre-mRNA splicing. In this paper, we show that Cdc40p is required at all temperatures for efficient entry into S-phase and that cell cycle arrest associated with cdc40 mutations is independent of all the known checkpoint mechanisms. Using immunofluorescence, we show that Cdc40p is localized to the nuclear membrane, weakly associated with the nuclear pore. Our results point to a link between cell cycle progression, pre-mRNA splicing, and mRNA export.     

Cbf5p, a potential pseudouridine synthase, and Nhp2p, a putative RNA-binding protein, are present together with Gar1p in all H BOX/ACA-motif snoRNPs and constitute a common bipartite structure

The eukaryotic nucleolus contains a large number of small nucleolar RNAs (snoRNAs) that are involved in preribosomal RNA (pre-rRNA) processing. The H box/ACA-motif (H/ACA) class of snoRNAs has recently been demonstrated to function as guide RNAs targeting specific uridines in the pre-rRNA for pseudouridine (psi) synthesis. To characterize the protein components of this class of snoRNPs, we have purified the snR42 and snR30 snoRNP complexes by anti-m3G-immunoaffinity and Mono-Q chromatography of Saccharomyces cerevisiae extracts. Sequence analysis of the individual polypeptides demonstrated that the three proteins Gar1p, Nhp2p, and Cbf5p are common to both the snR30 and snR42 complexes. Nhp2p is a highly basic protein that belongs to a family of putative RNA-binding proteins. Cbf5p has recently been demonstrated to be involved in ribosome biogenesis and also shows striking homology with known prokaryotic psi synthases. The presence of Cbf5p, a putative psi synthase in each H/ACA snoRNP suggests that this class of RNPs functions as individual modification enzymes. Immunoprecipitation studies using either anti-Cbf5p antibodies or a hemagglutinin-tagged Nhp2p demonstrated that both proteins are associated with all H/ACA-motif snoRNPs. In vivo depletion of Nhp2p results in a reduction in the steady-state levels of all H/ACA snoRNAs. Electron microscopy of purified snR42 and snR30 particles revealed that these two snoRNPs possess a similar bipartite structure that we propose to be a major structural determining principle for all H/ACA snoRNPs.     

The Hsp70 homologue Lhs1p is involved in a novel function of the yeast endoplasmic reticulum, refolding and stabilization of heat-denatured protein aggregates

Heat stress is an obvious hazard, and mechanisms to recover from thermal damage, largely unknown as of yet, have evolved in all organisms. We have recently shown that a marker protein in the ER of Saccharomyces cerevisiae, denatured by exposure of cells to 50 degrees C after preconditioning at 37 degrees C, was reactivated by an ATP-dependent machinery, when the cells were returned to physiological temperature 24 degrees C. Here we show that refolding of the marker enzyme Hsp150Delta-beta-lactamase, inactivated and aggregated by the 50 degrees C treatment, required a novel ER-located homologue of the Hsp70 family, Lhs1p. In the absence of Lhs1p, Hsp150Delta-beta-lactamase failed to be solubilized and reactivated and was slowly degraded. Coimmunoprecipitation experiments suggested that Lhs1p was somehow associated with heat-denatured Hsp150Delta- beta-lactamase, whereas no association with native marker protein molecules could be detected. Similar findings were obtained for a natural glycoprotein of S. cerevisiae, pro-carboxypeptidase Y (pro-CPY). Lhs1p had no significant role in folding or secretion of newly synthesized Hsp150Delta-beta-lactamase or pro-CPY, suggesting that the machinery repairing heat-damaged proteins may have specific features as compared to chaperones assisting de novo folding. After preconditioning and 50 degrees C treatment, cells lacking Lhs1p remained capable of protein synthesis and secretion for several hours at 24 degrees C, but only 10% were able to form colonies, as compared to wild-type cells. We suggest that Lhs1p is involved in a novel function operating in the yeast ER, refolding and stabilization against proteolysis of heatdenatured protein. Lhs1p may be part of a fundamental heat-resistant survival machinery needed for recovery of yeast cells from severe heat stress.     

Sro7p, a Saccharomyces cerevisiae counterpart of the tumor suppressor l(2)gl protein, is related to myosins in function

Yeast SRO7 was identified as a multicopy suppressor of a defect in Rho3p, a small GTPase that maintains cell polarity. Sro7p and Sro77p, a homologue of Sro7p, possess domains homologous to the protein that are encoded by the Drosophila tumor suppressor gene lethal (2) giant larvae [l(2)gl]. sro7Delta sro77Delta mutants showed a partial defect of organization of the polarized actin cytoskeleton and a cold-sensitive growth phenotype. A human counterpart of l(2)gl could suppress the sro7Delta sro77Delta defect. Similar to the l(2)gl protein, Sro7p formed a complex with Myo1p, a type II myosin. These results indicate that Sro7p and Sro77p are the yeast counterparts of the l(2)gl protein. Our genetic analysis revealed that deletion of SRO7 and SRO77 showed reciprocal suppression with deletion of MYO1 (i.e., the sro7Delta sro77Delta defect was suppressed by myo1Delta and vice versa). In addition, SRO7 showed genetic interactions with MYO2, encoding an essential type V myosin: Overexpression of SRO7 suppressed a defect in MYO2 and, conversely, overexpression of MYO2 suppressed the cold-sensitive phenotype of sro7Delta sro77Delta mutants. These results indicate that Sro7 function is closely related to both Myo1p and Myo2p. We propose a model in which Sro7 function is involved in the targeting of the myosin proteins to their intrinsic pathways.