Ca2+-ATPase protein expression in mammary tissue

Protein expression of plasma membrane Ca2+-ATPases (PMCAs) and the putative Golgi secretory pathway Ca2+-ATPase (SPCA) was examined in rat mammary tis sue. As lactation started, PMCA protein expression increased dramatically, and this increased expression paralleled milk production. Mammary PMCA was primarily PMCA2b but was similar to 4,000 daltons larger than expected. RT- PCR showed that the primary mammary PMCA2b transcript was alternatively spl iced, at splice site A, to include an additional 135 bp, resulting in the i nsertion of 45 amino acids. This splice form is designated 2bw. PMCA2bw is secreted into milk, associated with the milk fat globule membrane. Therefor e, PMCA2bw is located on the apical membrane of the secretory cell. Smaller amounts of PMCA1b and 4b protein were found in mammary tissue. PMCA4b was the major PMCA expressed in developing tissue, and its level declined as la ctation started. PMCA1b expression increased moderately during lactation. S PCA protein expression increased 1 wk before parturition and increased furt her as lactation proceeded. The abundance and cell location of PMCA2b sugge st that it is important for macro-Ca2+ homeostasis in lactating tissue. The pattern of expression and abundance of SPCA suggest that it is a candidate for the Golgi Ca2+-ATPase.