Immunochemical comparison of transformation-associated protein and secreted phosphoprotein

Transformation-associated protein (TAP) has been detected in MSV-M-transformed rat cell lines as glycosylated, weakly phosphorylated protein of molecular weight (Mr) 66,000 and 68,000. In the ts-MSV-M-transformed rat kidney cell line (6m2), the synthesis of TAP and the v-mos gene product is temperature-sensitive and accompanies the expression of transformation phenotypes. Therefore, TAP potentially plays a role in cellular transformation. On the other hand, SPP represents a family of glycosylated phosphoprotein with apparent Mr ranging from 42,000 to 69,000. SPP has been detected in osteoblasts and in avian and murine retrovirus-transformed rat and mouse epithelial cells. Therefore, the potential relatedness of TAP and SPP was studied. Using the 6m2 cells, we found that SPP was strongly phosphorylated and was synthesized at both the permissive (33 degrees C) and non-permissive (39 degrees C) temperatures. By contrast, TAP was weakly phosphorylated, and was synthesized, as we found previously, only at the permissive temperature of 33 degrees C. Furthermore, in 35S-methionine incorporation studies, TAP became heavily labelled whereas SPP was not (consistent with its amino acid composition having few methionine residues). Using 125I-TAP in both immunoprecipitation and radioimmunoassays, it was found that an antiserum raised against SPP did not cross-react with 125I-TAP. Additionally, SPP has now been found in many human and rodent cells, while TAP thus far has only been detected in MSV-transformed rat cells. These data suggest that structurally, TAP and SPP are not closely related phosphoproteins.