title: The elusive role of the N-terminal extension of beta A3- and beta A1-crystallin.
abstract: beta-Crystallins are structural lens proteins with a conserved two-domain structure and variable N- and C-terminal extensions. These extensions are assumed to be involved in quaternary interactions within the beta-crystallin oligomers or with other lens proteins. Therefore, the production of beta A3- and beta A1-crystallin from the single beta A3/A1 mRNA by dual translation initiation is of interest. These crystallins are identical, except that beta A1 has a much shorter N-terminal extension that beta A3. This rare mechanism has been conserved for over 250 million years during the evolution of the beta A3/A1 gene, suggesting that the generation of different N-terminal extensions confers a selective advantage. We therefore compared the stability and association behaviour of recombinant beta A3- and beta A1-crystallin. Both proteins are equally stable in urea- and pH-induced denaturation experiments. Gel filtration and analytical ultracentrifugation established that beta A3 and beta A1 both form homodimers. In the water-soluble proteins of bovine lens, beta A3 and beta A1 are present in the same molecular weight fractions, indicating that they oligomerize equally with other beta-crystallins. 1H-NMR spectroscopy showed that residues Met1 to Asn22 of the N-terminal extension of beta A3 have great flexibility and are solvent exposed, excluding them from protein interactions in the homodimer. These results indicate that the different N-terminal extensions of beta A3 and beta A1 do not affect their homo- or heteromeric interactions.
authors.affiliation: Department of Biochemistry, University of Nijmegen, The Netherlands.
authors.forename: P J
authors.initials: PJ
authors.lastname: Werten
authors.forename: J A
authors.initials: JA
authors.lastname: Carver
authors.forename: R
authors.initials: R
authors.lastname: Jaenicke
authors.forename: W W
authors.initials: WW
authors.lastname: de Jong
mesh_terms: D000595:Amino Acid Sequence; D005075:Biological Evolution; D003459:Crystallins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D008969:Molecular Sequence Data; D008970:Molecular Weight; D011485:Protein Binding; D011487:Protein Conformation; D011489:Protein Denaturation; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D017421:Sequence Analysis; D017386:Sequence Homology, Amino Acid; D013329:Structure-Activity Relationship
chemical_list: D003459:Crystallins; D011994:Recombinant Proteins