Transformation-specific secreted phosphoproteins

Abstract

We have previously shown that transformed mammalian cells secrete two classes of transformation-specific proteins of molecular weight (MW) about 60,000 (ref. 1). Secretion of both classes of proteins occurs regardless of the transforming agent and is temperature-sensitive in a cell line which is temperature-sensitive for the transformed phenotype (ts B77 Rat 1; see ref. 1). One of the classes consists of non-phosphorylated polypeptides. The proteins of this class, derived from transformed mouse, rat and hamster lines, are all antigenically related to one another. The second class of transformation-specific secreted proteins is antigenically distinct from the first class, that is, it is not precipitated by antiserum which recognizes the non-phosphorylated class. This second class comprises major phosphoproteins also in the 60,000 M W range¹. These major phosphoproteins were found to be secreted by transformed hamster, rat, mouse and rabbit cells. We now report that these transformation-specific phosphoproteins are all antigenically related, regardless of the transforming agent. The phosphoproteins are synthesized by the cells, phosphorylated intracellularly and subsequently released into the culture medium. These transformed cells also secrete protein kinase activities capable of phosphorylating the transformation-specific phosphoproteins in vitro. The major sites of in vivo and in vitro phosphorylation seem to be identical.