Homology modeling of Neurospora crassa geranylgeranyl pyrophosphate synthase: structural interpretation of mutant phenotypes |
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Authors: | Quondam M; Barbato C; Pickford A; Helmer-Citterich M; Macino G |
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Affiliation: | Dipartimento di Biopatologia Umana, Policlinico Umberto I, Universita di Roma La Sapienza, Rome, Italy. |
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Abstract: | A model of the tertiary structure of the Neurospora crassa carotenogenic
prenyltransferase, geranylgeranyl pyrophosphate synthase (GGPPS), is
presented, based on structural homology with other prenyltransferases and
on the crystal structure of recombinant avian farnesyl pyrophosphate
synthase (FPPS). The conserved aspartate-rich motifs DDxx(xx)D and
associated basic residues, considered to be the active sites for binding
and catalysis in all prenyltransferases, are highly conserved in the N.
crassa GGPPS protein, while other regions display a lower degree of
sequence homology; thus the GGPPS model structure is predicted to be highly
reliable in the active site region. A number of carotene-deficient mutants
have been generated utilizing the repeat-induced point mutation (RIP)
mechanism: mutant al-3RIP1 carries a Ser-to-Asn mutation in position 336
which falls within the predicted active site of the enzyme. Analysis of the
model structure of this mutant indicates that Ser336 may be involved in
substrate uptake. Two other mutants, al-3RIP3 and al-3RIP6, carry mutations
in positions in the GGPPS protein, homologous to regions of the avian FPPS
enzyme proposed to be involved in enzyme dimerization and substrate uptake,
respectively, suggesting an explanation for the reduced carotene content of
these mutants.
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