title: Calpactins: two distinct Ca++-regulated phospholipid- and actin-binding proteins isolated from lung and placenta.
abstract: Three forms of calpactin, the 36,000 Mr Ca++-binding cytoskeletal protein, were isolated in large amounts from bovine lung and human placenta using cycles of calcium-dependent precipitation followed by solubilization with EGTA-containing buffers. Calpactin-I as a tetramer of heavy (36 kD) and light (11 kD) chains was the predominant form of calpactin isolated, however milligram amounts of the calpactin-I heavy chain monomer and calpactin-II, a related but distinct molecule, were also isolated by this method. Calpactin-II was characterized in some detail and found to bind two Ca++ ions with Kd's of 10 microM in the presence of phosphatidylserine. Both calpactin-I and -II were found to aggregate liposomes at micromolar Ca++ concentrations, suggesting that at least two phospholipid-binding sites are present on these molecules. Both calpactin monomers bind to and bundle actin filament at high (1 mM) but not low (less than 1 microM) Ca++ concentrations. Amino-terminal sequence analysis of a lower molecular mass variant of calpactin-II revealed that this protein was the previously identified human "lipocortin" molecule. Antibodies were elicited to calpactin-I and -II and the cell and subcellular distribution of each was compared. Calpactin-II was only present at high levels in tissues (lung, placenta) which contained high levels of calpactin-I. Other tissues (intestine) contained high calpactin-I and undetectable levels of calpactin-II. Double-label immunofluorescence microscopy on human fibroblasts revealed that, like calpactin-I, calpactin-II is present in a submembraneous reticular network, although the distribution of the two calpactins is not identical.
mesh_terms: D000199:Actins / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D017302:Annexins; D002118:Calcium / Q000378:metabolism; D002135:Calcium-Binding Proteins / Q000302:isolation & purification* / Q000378:metabolism; D002417:Cattle; D005260:Female; D006801:Humans; D007700:Kinetics; D008168:Lung / Q000378:metabolism*; D046911:Macromolecular Substances; D008970:Molecular Weight; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D011485:Protein Binding
chemical_list: D000199:Actins; D017302:Annexins; D002135:Calcium-Binding Proteins; D046911:Macromolecular Substances; D002118:Calcium