CLONING OF THE ALPHA-1 SUBUNIT OF A VOLTAGE-DEPENDENT CALCIUM-CHANNEL EXPRESSED IN PANCREATIC BETA-CELLS

The isoforms of the alpha-1 subunits of voltage-dependent Ca2+ channels expressed in human pancreatic islets were identified by using a pair of degenerate oligonucleotide primers and the polymerase chain reaction (PCR) to amplify mRNAs encoding alpha-1 subunit-like sequences. The sequences of the PCR products indicate that islets express the heart-type alpha-1 subunit as well as a second isoform whose complete sequence has not been previously reported. The sequences of cloned cDNAs encoding the human beta-cell, or neuroendocrine-type, alpha-1, subunit indicate that it is composed of 2181 amino acids. It shares 68 %, 64 %, and 41 % identity with the sequences of the alpha-1 subunits of rabbit heart, skeletal muscle, and brain, respectively, and is predicted to have a similar structure including four homologous domains composed of six membrane-spanning segments each. RNA blotting studies indicate that the beta-cell-type alpha-1 subunit is also expressed in brain as well as in the insulin-producing cell lines RINm5F and beta-TC-3; however, it could not be detected by RNA blotting in a third cell line, HIT-T15. In situ hybridization studies revealed expression of beta-cell-type alpha-1 subunit mRNA in beta-cells of rat pancreatic islets, implying that this protein may play a role in the regulation of insulin secretion.