Identification and characterization of beta V spectrin, a mammalian ortholog of Drosophila beta(H) spectrin

Four mammalian beta-spectrin genes are currently recognized, all encode pro teins of approximate to 240-280,000 M-r and display 17 triple helical homol ogous approximate to 106-residue repeat units. In Drosophila and Caenorhabd itis elegans, a variant beta spectrin with unusual properties has been reco gnized. Termed beta heavy (beta(H)), this spectrin contains 30 spectrin rep eats, has a molecular weight in excess of 400,000, and associates with the apical domain of polarized epithelia, We have cloned and characterized from a human retina cDNA library a mammalian ortholog of Drosophila beta(H) spe ctrin, and in accord with standard spectrin naming conventions we term this new mammalian spectrin beta 5 (beta V) The gene for human beta V spectrin (HUBSPECV) is on chromosome 15q21. The 11,722-nucleotide cDNA of beta V spe ctrin is generated from 68 exons and is predicted to encode a protein with a molecular weight of 416,960. Like its fly counterpart, the derived amino acid sequence of this unusual mammalian spectrin displays 30 spectrin repea ts, a modestly conserved actin-binding domain, a conserved membrane associa tion domain 1, a conserved self-association domain, and a pleckstrin homolo gy domain near its COOH terminus. Its putative ankyrin-binding domain is po orly conserved and may be inactive. These structural features suggest that beta V spectrin is likely to form heterodimers and oligomers with alpha spe ctrin and to interact directly with cellular membranes. Unlike its Drosophi la ortholog, beta V spectrin does not contain an SH3 domain but displays in repeat 5 a 45-residue insertion that displays 42% identity to amino acids 85-115 of the E4 protein of type 75 human papilloma virus. Human beta V spe ctrin is expressed at low levels in many tissues. By indirect immunofluores cence, it is detected prominently in the outer segments of photoreceptor ro ds and cones and in the basolateral membrane and cytosol of gastric epithel ial cells. Unlike its Drosophila ortholog, a distinct apical distribution o f beta V spectrin is inapparent in the epithelial cell populations examined , although it is confined to the outer segments of photoreceptor cells. The complete cDNA sequence of human beta V spectrin is available from GenBank( TM) as accession number AF233523.