The dihydropyridine receptor (DHPR) of skeletal muscle is a voltage dependent L-type Ca2+ channel that controls the entry of Ca2+ into cells to regulate the activity of the channel itself and several Ca2+ dependent signal transduction pathways. The DHPR is also a mechanical gate for the Ca2+ release channel (the ryanodine receptor, RYR1) that regulates release of Ca2+ from intracellular stores to initiate contraction. Physical coupling between the DHPR and RYR1 drives RYR1 activation (orthograde coupling) and prevents L-type channel inactivation (retrograde coupling). We have shown that one site on the L-type channel that interacts with RYR1 is the carboxyterminal tail of the alpha1 subunit. This region of the L-type channel of skeletal and cardiac muscle also binds both Ca2+ and calmodulin (CAM). Ca2+ binding to the bound CaM produces Ca2+ dependent inactivation (CDI) of the channel. Our long range goals are to determine the structure of the DHPR and elucidate the features of the carboxyterminal tails of both the cardiac and skeletal L-type channels that contribute to CDI and E-C coupling. Our specific aims are to: 1) Determine the structure of the DHPR by cryoelectron microscopy and image reconstruction and map the location of the carboxyterminal tail and the alpha2, beta and gamma subunits in the 3D structure; 2) Elucidate the structure of the carboxyterminal tail of the DHPR alpha1 subunit complexed to CaM; and 3) Evaluate the ability of the carboxyterminal tail of the alpha1 subunit of the DHPR to bind Ca2+ and CaM binding motifs. [unreadable] [unreadable]