Cleavage of reduced, carboxymethylated, delipidated CA2+-transporting ATPase protein from rabbit sarcoplasmic reticulum with dimethyl sulphoxide/HBr yielded two long peptides (38 and 73 residues), distinct from the known major sequences of the ATPase. The longer peptide contained at least two cysteine residues, which were disulphide-linked in the native protein. It was therefore derived from the B-fragment of the ATPase in which the disulphides had previously been located. It probably formed a loop on the luminal side of the membrane, spanning two membrane-buried tryptophan residues. The N-terminal sequence of this peptide, (Trp)-Phe-Met-Tyr-Ala, forms the basis for an oligodeoxynucleotide probe, the use of which to identify cDNA corresponding to the ATPase is described elsewhere [MacLennan, Brandl, Korczak & Green (1985) Nature (London) 316, 696-700].

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