Molecular Cloning of the Mammalian Smooth Muscle Sarco(endo)plasmic Reticulum Ca2+-ATPase

We have isolated and sequenced full-length cDNA clones from a rabbit uterine library which encode the smooth muscle sarco(endo)plasmic reticulum Ca2+-ATPase. These cDNAs resulted from an alternative splice of the cardiac/slow-twitch Ca2+-ATPase gene transcript, and encoded a protein identical to rabbit cardiac/slow-twitch Ca2+-ATPase except for the replacement of the carboxyl-terminal four amino acids with an extended and relatively hydrophobic sequence of 49 amino acids. This cDNA was virtually identical to the alternatively spliced product of the cardiac/slow-twitch Ca2+-ATPase gene recently identified in human kidney (Lytton, J., and MacLennan, D. H. (1988) J. Biol. Chem. 263, 15024-15031) and rat non-muscle tissues (Gunteski-Hamblin, A.-M., Greeb, J., and Shull, G. (1988) J. Biol. Chem. 263, 15032-15040). S1 nuclease mapping of total cellular RNA from a variety of tissues demonstrated that cardiac muscle expressed the cardiac/slow-twitch isoform almost exclusively, most smooth muscle and non-muscle tissues expressed the alternatively spliced smooth/non-muscle isoform almost exclusively, and a few tissues expressed both isoforms in varying amounts. Thus, regulation of alternative splicing of the cardiac/slow-twitch Ca2+-ATPase gene transcript is tissue-specific. The expression of the smooth/non-muscle isoform in every tissue tested supports the hypothesis that this molecule represents the "housekeeping" endoplasmic reticulum Ca2+-ATPase.