Voltage-activated calcium channels regulate the entry of calcium into the cytoplasm of excitable cells and thereby mediate numerous activities in the nervous system. Molecular characterization of these channels is an important step in understanding how ion channels function and how their activities are regulated. To a large extent, molecular studies of calcium channels have focussed on the dihydropyridine (DHP) binding component found in skeletal muscle. Purified DHP receptor contains a large 200 kDa polypeptide (alpha 1) that has the structural and functional properties expected for the DHP-sensitive calcium channel. Three other polypeptides (alpha 2, beta, and gamma) are also found in these purified preparations but whether they are functional components of the calcium channel or adventitiously associated proteins is unknown. By using antibodies specific for each of the polypeptides, we will determine if these accessory proteins are always associated with the central ion channel alpha 1 protein by comparing their localization in muscle, their tissue distributions, and their expression during skeletal muscle development. An alpha 1-like protein can also be isolated from cells derived from human small cell carcinoma of the lung (SCCL). SCCL is frequently associated with Lambert-Eaton Syndrome (LES), a neuromuscular disease characterized by deficient release of acetylcholine. This deficiency may result from an antibody-induced reduction in the number of functional calcium channels. We will test the hypothesis that the SCCL alpha 1-like protein is a calcium channel that elicits an autoimmune response to the nerve terminal channel. cDNA clones encoding the SCCL alpha 1-like protein will be isolated and sequenced to determine if this protein has the structure expected of a voltage-regulated calcium channel. Mice will be injected with the alpha- like protein expressed in a bacterial system or with monoclonal antibodies that recognize it to test their ability to induce LES. These studies may identify the proteins responsible for a neuromuscular autoimmune disease and, in addition, provide antibodies that may be useful in the diagnosis and treatment of SCCL.
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