Identification and characterization of βV spectrin, a mammalian ortholog of Drosophila β(H) spectrin

Abstract

Four mammalian β-spectrin genes are currently recognized, all encode proteins of κ240-280,000 M(r) and display 17 triple helical homologous κ106-residue repeat units. In Drosophila and Caenorhabditis elegans, a variant β spectrin with unusual properties has been recognized. Termed β heavy (β(H)), this spectrin contains 30 spectrin repeats, 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 β(H) spectrin, and in accord with standard spectrin naming conventions we term this new mammalian spectrin β 5 (βV). The gene for human βV spectrin (HUBSPECV) is on chromosome 15q21. The 11,722-nucleotide cDNA of βV spectrin 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 repeats, a modestly conserved actin- binding domain, a conserved membrane association domain 1, a conserved self- association domain, and a pleckstrin homology domain near its COOH terminus. Its putative ankyrin-binding domain is poorly conserved and may be inactive. These structural features suggest that βV spectrin is likely to form heterodimers and obligomers with α spectrin and to interact directly with cellular membranes. Unlike its Drosophila ortholog, βV 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 βV spectrin is expressed at low levels in many tissues. By indirect immunofluorescence, it is detected prominently in the outer segments of photoreceptor rods and cones and i the basolateral membrane and cytosol of gastric epithelial cells. Unlike its Drosophila ortholog, a distinct apical distribution of βV 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 βV spectrin is available from GenBank(TM) as accession number AF233523.