MOLECULAR-CLONING AND TISSUE DISTRIBUTION OF ALTERNATIVELY SPLICED MESSENGER-RNAS ENCODING POSSIBLE MAMMALIAN HOMOLOGS OF THE YEAST SECRETORY PATHWAY CALCIUM-PUMP

Rat stomach and testis cDNAs corresponding to two alternatively spliced mRNAs encoding variants of a P-type ion-transport ATPase that closely resembles the yeast secretory pathway Ca2+ pump have been isolated and characterized. A partial kidney cDNA was identified previously using an oligonucleotide probe corresponding to part of the sarcoplasmic reticulum Ca2+-ATPase [Gunteski-Hamblin, A., Greeb, J., & Shull, G. E. (1988) J. Biol. Chem. 263, 15032-15040]. In the present study, we first isolated and characterized a stomach cDNA that contains the entire coding sequence. The 919 amino acid enzyme has the same apparent transmembrane organization and contains all of the conserved domains present in other P-type ATPases. Northern blot analyses demonstrate that 3.9- and 5-kilobase mRNAs corresponding to the cDNA were present in all tissues examined, suggesting that the protein it encodes performs a housekeeping function. Rat testis also contained a 3.7-kilobase mRNA that hybridized with a probe from the 5' end of the stomach cDNA but did not hybridize with a probe from the 3' end. Cloning and characterization of cDNAs corresponding to the smaller testis mRNA revealed that it is derived from the same gene but encodes a variant of the enzyme in which the C-terminal residue, Val-919, is replaced by the sequence Phe-919-Tyr-Pro-Lys-Ile-923. Similarity comparisons show that the two enzymes are more closely related to the known Ca2+ pumps than to other P-type ATPases. They exhibit 23% amino acid identity with the plasma membrane Ca2+-ATPase, 33% identity with the sarcoplasmic reticulum Ca2+-ATPase, and 50% identity with the yeast secretory pathway Ca2+-ATPase. On the basis of these comparisons, it seems likely that they are mammalian homologues of the yeast secretory pathway Ca2+ pump.