MODIFICATION OF CYS-837 IDENTIFIES AN ACTIN-BINDING SITE IN THE BETA-PROPELLER PROTEIN SCRUIN

In the acrosomal process of Limulus sperm, the beta-propeller protein scruin cross-links actin into a crystalline bundle. To confirm that sc ruin has the topology of a beta-propeller protein and to understand ho w scruin binds actin, we compared the solvent accessibility of cystein e residues in scruin and the acrosomal process by chemical modificatio n with (1,5-IAEDANS). In soluble scruin, the two most reactive cystein es of soluble scruin are C837 and C900, whereas C146, C333, and C683 a re moderately reactive. This pattern of reactivity is consistent with the topology of a typical beta-propeller protein; all of the reactive cysteines map to putative loops and turns whereas the unreactive cyste ines lie within the predicted interior of the protein. The chemical re activities of cysteine in the acrosomal process implicate C837 at an a ctin-binding site. In contrast to soluble scruin, in the acrosomal pro cess, C837 is completely unreactive while the other cysteines become l ess reactive. Binding studies of chemically modified scruin correlate the extent of modification at C837 with the extent of inhibition of ac tin binding. Furthermore, peptides corresponding to residues flanking C837 bind actin and narrow a possible actin-binding region to a KQK se quence. On the basis of these studies, our results suggest that an act in-binding site lies in the C-terminal domain of scruin and involves a putative loop defined by C837.