Interactions in vitro of p9Ka, the rat S-100-related, metastasis-inducing, calcium-binding protein.

The S-100 proteins are a structurally related family displaying diverse intracellular and extracellular interactions. One such protein p9Ka (also known as calvasculin), or its mRNA (also known as CAPL, 42A, 18A2, mts, pEL 98), becomes elevated upon changes in the growth and differentiation of cells. Overexpression of p9Ka in benign rat mammary cells induces the metastatic phenotype. In order to help understand the role of p9Ka in these processes, the molecular properties of recombinant rat p9Ka have been studied. Recombinant p9Ka forms multimers in vitro, which are not due to intermolecular disulfide bridges, it binds 2 mol of calcium ions/mol of protein, and the binding of calcium ions is strongly antagonized by monovalent and divalent cations tested. Immunofluorescence studies indicate that p9Ka is located on cytoskeletal elements in a pattern which is identical to actin filaments stained with phalloidin. In vitro, it is shown that recombinant p9Ka binds to sites on at least two intracellular polypeptides. These sites display the same binding capacity for p9Ka in extracts of cultured rat mammary cells which show widely differing levels of expression of natural p9Ka. The results suggest that the production of p9Ka, and not of its target molecules, may be associated with the changes seen in cultured cells.