The elusive role of the N-terminal extension of {beta}A3- and {beta}Al-crystallin |
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Authors: | Werten, Paul J.L. Carver, John A. Jaenicke, Rainer de Jong, Wilfried W. |
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Affiliation: | 1Department of Biochemistry, University of Nijmegen 6500 HB Nijmegen, The Netherlands 2Australian Cataract Research Foundation, Department of Chemistry, University of Wollongong Wollongong, Australia 3Institut für Biophysik und Physikalische Biochemie D-93053 Regensburg, Germany |
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Abstract: | ß-Crystallins are structural lens proteins with aconserved two-domain structure and variable N- and C-terminalextensions. These extensions are assumed to be involved in quaternaryinteractions within the ß-crystallin oligomers orwith other lens proteins. Therefore, the production of ßA3-and ßAl-crystallin from the single ßA3/A1mRNA by dual translation initiation is of interest. These crystallinsare identical, except that ßAl has a much shorterN-terminal extension than ßA3. This rare mechanismhas been conserved for over 250 million years during the evolutionof the ßA3/A1 gene, suggesting that the generationof different N-terminal extensions confers a selective advantage.We therefore compared the stability and association behaviourof recombinant ßA3- and ßAl-crystallin.Both proteins are equally stable in urea- and pH-induced denaturationexperiments. Gel filtration and analytical ultracentrifugationestablished that ßA3 and ßA1 both form homodimers.In the water-soluble proteins of bovine lens, ßA3and ßA1 are present in the same molecular weight fractions,indicating that they oligomerize equally with other ß-crystallins.1H-NMR spectroscopy showed that residues Met1 to Asn22 of theN-terminal extension of ßA3 have great flexibilityand are solvent exposed, excluding them from protein interactionsin the homodimer. These results indicate that the differentN-terminal extensions of ßA3 and ßA1 donot affect their homo- or heteromeric interactions. |
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Keywords: | ß -crystallin/ lens proteins/ protein association/ protein evolution/ sequence extensions |
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