The lens protein alpha A-crystallin of the blind mole rat, Spalax ehrenbergi: evolutionary change and functional constraints.

Abstract

The complete structure of the single-copy aA-crystallin gene of the blind mole rat (Spalax ehrenbergi) has been determined in order to elucidate the evolutionary effects of the loss of vision on a lens-specific protein and its gene. The aA-crystallin gene appears to have all the necessary transcriptional and translational signal sequences to be expressed in the rudimentary lens of the mole rat and gives rise to probably two protein products by means of alternative splicing, as in rodents with normal vision. Comparisons of the blind mole rat aAcrystallin sequence with aA sequences from other rodents reveal a considerable acceleration of the substitution rate at nonsynonymous positions in the mole rat lineage, which reflects a relaxation of selective constraints, but the acceleration is not to the extent that might be expected if the gene were now without any function. The remaining evolutionary constraints still imposed upon the mole rat aA-crystallin gene may possibly reflect the need for a-crystallin expression as an indispensable component in the developmental program of the atrophied eye.Functional constraints working at the protein level are recognized as a major determinant of the rate of molecular evolution (1-3). Although several well-known examples clearly seem to illustrate this principle, such as the fastevolving fibrinopeptides versus the conserved histones (4), the structure-function relationships of most proteins are, in fact, not understood in sufficient detail to reliably correlate functional constraint with rate of evolution. This difficulty may also hamper the clear distinction of other types of constraints, such as amino acid composition (5), transcriptional and translational requirements (6), and developmental programs (7,8). The existence and importance of these additional selective forces might be revealed by studying genes whose protein products have lost their normal principal function in the course of evolution. The substitution rate of such a gene would be expected to increase and might eventually become as fast as in pseudogenes unless other constraints interfere. A unique opportunity for such a study is offered by the rudimentary eyes of blind vertebrate species. The eye-specific genes of these animals are no longer subject to the selective constraints associated with the maintenance of vision.The eye lens protein a-crystallin is very suitable for this purpose because much comparative and evolutionary data are available (9-11). a-Crystallin is a slowly evolving protein (10), and, thus, relaxation of constraints should be readily detectable. An important advantage, too, is the fact that the atA and aB subunits, of which at-crystallin is composed, are both encoded by single-copy genes located on different chromosomes (12). The aA and aB chains have =57% sequence homology, due to an ancient gene duplication (10), and conspicuous sequence similarity with the small heat shock proteins indicates that the ancestral ca-crystallin gene originated from this protein family (13,14). aA-Cry...

Cloning and characterisation of the gene encoding mole rat (Spalax ehrenbergi) growth hormone

“…The sequence of mature mole rat GH can be usefully compared with that of the putative ancestral placental mammal GH (Wallis 1994), which is identical to the sequences of pig and dog GH (Vize & Wells 1987, AscasioMartinez & Barrera-Saldana 1994. Mole rat GH differs from this slightly less than do other rodent GHs for which sequences are available and the pattern of variation observed strongly supports other molecular and fossil evidence suggesting an early evolutionary divergence time for Spalacidae from the muroid-cricetoid stock (Kleinschmidt et al 1985, Hendriks et al 1987, Nevo 1991, Catzeflis et al 1993. Use of the ancestral sequence allows the numbers of substitutions introduced into GH during various stages of rodent evolution to  3.…”

Adaptive evolution of small heat shock protein/ αB-crystallin promoter activity of the blind subterranean mole rat,Spalax ehrenbergi

“…Despite that the mole rat ␣A-crystallin cDNA has undergone numerous mutations, consistent with its lack of use as a crystallin in the degenerated lens of this species (25), the sequence of the mole rat ␣A-crystallin promoter is surprisingly similar to that of the mouse (9,25). The ␣A-crystallin promoter sequence of the blind cavefish is also highly similar to the mouse despite that the gene is not expressed in the lens (26).…”

Expression of the murine alpha B-crystallin gene is not restricted to the lens.

“…While there is no evidence demonstrating their response to heat shock in lens (9, 10), Duguid and co-workers (12) have recently suggested that oxB-crystallin may play a role in response to cellular stress in nonlenticular tissue. It was previously suggested, based on the slow rate of otA-crystallin evolution in the blind mole rat, that this crystallin might play some role other than that of a structural component of the lens (17,30 of its expression in a variety of tissues makes the search for its cellular role even more essential.…”