The human desmin and vimentin genes are located on different chromosomes

We have used somatic cell hybrids of Chinese hamster X man and mouse X man to localize the genes (des and vim) encoding the intermediate filaments desmin and vimentin in the human genome. Southern blots of DNA prepared from each cell line were screened with hamster cDNA probes specific for des and vim genes, respectively. The single-copy human des gene is located on chromosome 2, and the single-copy human vim gene is assigned to chromosome 10. Partial restriction maps of the two human genomic loci are presented. A possible correlation of the des locus with several reported hereditary myopathies is discussed.     

Evolution of crystallins: expression of lens-specific proteins in the blind mammals mole (Talpa europaea) and mole rat (Spalax ehrenbergi)

The mole (Talpa europaea; Insectivora) and the mole rat (Spalax ehrenbergi; Rodentia) both have degenerated eyes as a convergent adaptation to subterranean life. The rudimentary eye lenses of these blind mammals no longer function in a visual process. The crystallin genes, which display a lens-specific expression pattern, were studied in these blind mammals and in related species with normal eyes by hybridizing their genomic DNAs with probes obtained from cDNA clones for alpha A-, alpha B-, and beta Bp-crystallins from calf and gamma 3- crystallin from the rat. For all crystallin genes examined, the hybridization signals of mole and mole rat genomic DNA were comparable, respectively, with those of shrew and of rat and mouse, normal-vision representatives of the orders Insectivora and Rodentia. The expression of the crystallins at the protein level was tested by using antiserum specific for alpha-crystallin in immunofluorescence reactions on lens sections of mole and mole rat eyes and by using antisera against the beta- and gamma-crystallins on sections of the mole eye. All antisera gave positive fluorescence reactions exclusively with lens tissue of these blind mammals, indicating that the crystallins are still normally expressed despite the fact that these lenses have had no function in a visual process in these mammals for at least many million years. These findings apparently imply that some unknown selective advantage has conserved the crystallin genes and their expression after the loss of normal function of the lenses.      

beta s-Crystallin: structure and evolution of a distinct member of the beta gamma-superfamily

The nucleotide sequence of the cDNA of bovine lens beta s-crystallin has been determined, and the derived amino acid sequence has been confirmed by amino acid compositions and partial sequences of the tryptic peptides of this monomeric protein. beta s-Crystallin has a length of 177 residues, corresponding to a mol. wt. of 20 773, and a blocked N-terminal serine. Comparison of beta s with the known sequences of other beta- and gamma-crystallins, and computer construction of a phylogenetic tree of these sequences, shows beta s to be more closely related to the monomeric gamma-crystallins than to the oligomeric beta-crystallins. Also the tertiary structure of beta s modelled by interactive computer graphics on the coordinates of gamma II-crystallin, revealed similarities with the gamma-crystallins which might explain its monomeric behavior: the presence of a very short N-terminal 'arm' as compared with the beta-crystallins; a distribution of charged residues on the surface as in the gamma-crystallins; and finally the nature of certain residues of its inter-domain contacts. beta s-Crystallin seems to be an old and isolated offshoot of the gamma-family, and, considering its ancient origin, might well be present in other, non-mammalian, vertebrate classes.     

Bovine β-crystallin complementary DNA clones: Alternating proline/alanine sequence of βB1 subunit originates from a repetitive DNA sequence

A library of recombinant plasmids carrying complementary DNA sequences synthesized from bovine lens messenger RNAs was constructed. Clones coding for five different β-crystallin subunits: βB1, βB3, βBp, βs, βA3 (and βA1), were identified by means of hybridization selection, followed by one- and two-dimensional gel electrophoresis of the translational products. Under rather stringent conditions each of these clones hybridizes with its corresponding mRNA and does not show significant cross-hybridization with mRNAs coding for other β-crystallins, except in the case of the homologous βA3 and βA1-crystalline. The βA3 and βA1 subunits seem to be encoded by one mRNA using two different AUG codons as start position for translation. We have also determined the nucleotide sequence of a βB1-crystallin cDNA (pBLβB1) which enabled us to deduce the complete amino acid sequence of the protein. The βB1-crystallin, a characteristic component of the high molecular weight crystallin aggregate (β_H), is internally homologous both at DNA and protein level as has been reported for γ-and other β-crystallins. This is in agreement with the idea that these proteins had a common ancestral precursor gene that internally duplicated. The G + C content of the coding sequence of βB1 is very high: 67% overall and even 84.2% for the first 170 nucleotides, due to a remarkable non-random codon usage. A proline/ alanine repetition in the N-terminal domain of the protein is encoded by a repetitive “simple” DNA sequence.     

Organization and expression of the vimentin gene

The primary structure of hamster vimentin has been derived from the nucleotide sequence of the corresponding cDND. The elucidation of the amino acid sequence allowed the prediction of a model in which two helix domains occur. The N-terminal and C-terminal part have been characterized as nonhelical domains. Moreover the two helices are separated by a third nonhelical region.     

The structure of the vimentin gene

The structure of the chromosomal gene encoding the intermediate filament protein vimentin is described. This gene, which is present as a single copy in the hamster genome, comprises about 10 kb of DNA and contains more than 80% of intron sequences. S1 mapping and sequence analysis reveal nine exons with a total length of 1848 nucleotides. For the complete primary structure of hamster vimentin, 464 amino acids are predicted, giving a molecular weight of 53,500 daltons. The intron positions are at codons 186, 206/207, 238/239, 292/293, 334/335, 408, 423, and 451/452. The overall homology with chicken desmin is 60% and is even higher in the central (alpha-helical) regions of both molecules. Cross-hybridization at the DNA level, however, is low. Comparison of the amino acid sequence of vimentin with prekeratin sequences shows that there is lesser homology of primary structure, but both the position and size of alpha-helical regions are strongly conserved. At the 5' end of the gene there is a consensus promoter sequence. The first AUG start codon is found 132 nucleotides downstream of the estimated cap site. The 3' nontranslated sequence shows homologies with the chicken vimentin gene. An interesting feature of the vimentin gene is a stretch of 44 nucleotides of alternating dC and dA within intron 2 that may form left-handed Z-DNA.