An analog of omega-conotoxin MVIIC (Y13A-MVIIC) was synthesized by replacing Tyr(13) with Ala to study the role of Tyr(13) residue conserved in many omega-conotoxins. Y13A-MVIIC has an overall conformation similar to that of the native toxin, but an enormously reduced ability to displace I-125-omega-conotoxin MVIIC binding to rat cerebellar P2 membranes. These results suggest that Tyr(13) is essential for the activity of omega-conotoxins at P/Q-type calcium channels. (C) 1995 Academic Press, Inc.