DEFINITION OF A SEQUENCE UNIQUE IN BETA-II SPECTRIN REQUIRED FOR ITS AXON-SPECIFIC INTERACTION WITH FODAXIN (A60)

Spectrin isotypes segregate in neurons and are differentially distribu ted between axons and somatodendritic compartments. Their functions in those compartments are likely to be mediated by proteins that interac t selectively with one or other isotype. Fodaxin (an axon-specific pro tein previously termed A60) colocalizes in CNS neurons with axonal spe ctrin and in vitro binds brain spectrin (a mixture of alpha I, beta I, alpha II, and beta II polypeptides) but not erythrocyte spectrin (alp ha I and beta I). Because alpha II and beta II spectrin polypeptides a re enriched in axons, we investigated a possible binding of fodaxin to the types of spectrin found in axons. Fodaxin did not bind to isolate d brain alpha chains. Bacterially expressed C-terminal segments 18-19 of beta II spectrin bound to fodaxin and inhibited the binding of foda xin to whole brain spectrin. By contrast, recombinant segments 18-19 o f the somatodendritic beta I Sigma 2 spectrin showed no interaction wi th fodaxin. Within beta II, fodaxin binding activity was localized to residues 2,087-2,198, which are unique to beta II and link between the end of segment 18 and the pleckstrin homology domain in segment 19. T he divergent regions of sequence in segments 19 of beta II and beta I Sigma 2 are candidates to mediate the isotype-specific functions of sp ectrin. Fodaxin is the first protein to be described that discriminate s between the unique regions of beta spectrin isoforms.