A WIDELY EXPRESSED BETA-III SPECTRIN ASSOCIATED WITH GOLGI AND CYTOPLASMIC VESICLES

Spectrin is an important structural component of the plasma membrane s keleton. Heretofore-unidentified isoforms of spectrin also associate w ith Golgi and other organelles. We have discovered another member of t he P-spectrin gene family by homology searches of the GenBank database s and by 5' rapid amplification of cDNA ends of human brain cDNAs. Col lectively, 7,938 nucleotides of contiguous clones are predicted to enc ode a 271,293-Da protein, called pm spectrin, with conserved actin-, p rotein 4.1-, and ankyrin-binding domains, membrane association domains 1 and 2, a spectrin dimer self-association site, and a pleckstrinhomo logy domain. beta III spectrin transcripts are concentrated in the bra in and present in the kidneys, liver, and testes and the prostate, pit uitary, adrenal, and salivary glands. All of the tested tissues contai n major 9.0-kb and minor 11.3-kb transcripts. The human pm spectrin ge ne (SPTBN2) maps to chromosome 11q13 and the mouse gene (Spnb3) maps t o a syntenic region close to the centromere on chromosome 19. Indirect immunofluorescence studies of cultured cells using antisera specific to human pm spectrin reveal a Golgi-associated and punctate cytoplasmi c vesicle-like distribution, suggesting that beta III spectrin associa tes with intracellular organelles. This distribution overlaps that of several Golgi and vesicle markers, including mannosidase II, p58, tran s-Golgi network (TGN)38, and beta-COP and is distinct from the endopla smic reticulum markers calnexin and Bip. Liver Golgi membranes and oth er vesicular compartment markers cosediment in vitro with beta III spe ctrin. beta III spectrin thus constitutes a major component of the Gol gi and vesicular membrane skeletons.