The calpactin light chain is tightly linked to the cytoskeletal form of calpactin I: studies using monoclonal antibodies to calpactin subunits.
Calpactins are a family of related Ca++-regulated cytoskeletal proteins. To analyze the expression and cytoskeletal association of calpactins we raised monoclonal antibodies with specificity for the heavy or light chains of calpactin I or to calpactin II. Comparison of the tissue distribution of calpactin I heavy and light chains by Western blots revealed that these subunits are coordinately expressed. Both soluble and cytoskeletal forms of the heavy chain of calpactin I were detected in human fibroblasts whereas only a soluble pool of calpactin II was found. These two forms of the calpactin I heavy chain differed both in their state of association with the light chain and in their rate of turnover. Both the soluble pool of the calpactin I heavy chain and calpactin II turned over three to four times faster than the cytoskeletal pool of heavy and light chains. Immunofluorescence microscopy revealed that the calpactin I light chain was present exclusively in the cytoskeleton whereas the calpactin I heavy chain distribution was more diffuse. No difference in the amount of light chain or the cytoskeletal attachment of phosphorylated calpactin I heavy chain was found in Rous sarcoma virus-transformed chick embryo fibroblasts compared with their normal counterpart. The antibody to the light chain of calpactin I was microinjected into cultured fibroblasts and kidney epithelial cells. In many cases antibody clustering was observed with the concomitant aggregation of the associated calpactin I heavy chain. The distribution of fodrin and calpactin II in injected cells remained unchanged. These results are consistent with the existence of two functionally distinct pools of calpactin I which differ in their association with the cytoskeleton.
D000818:Animals; D017302:Annexins; D000911:Antibodies, Monoclonal; D000936:Antigen-Antibody Complex / Q000032:analysis; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D002417:Cattle; D003599:Cytoskeleton / Q000378:metabolism* / Q000648:ultrastructure; D005347:Fibroblasts / Q000166:cytology / Q000378:metabolism; D005455:Fluorescent Antibody Technique; D006801:Humans; D007700:Kinetics; D008168:Lung / Q000378:metabolism; D046911:Macromolecular Substances
D017302:Annexins; D000911:Antibodies, Monoclonal; D000936:Antigen-Antibody Complex; D002135:Calcium-Binding Proteins; D046911:Macromolecular Substances