A 56,000 M(R) PHOSPHOSERYL PROTEIN IN PC12 CELL LYSATES STRONGLY ASSOCIATES WITH PROTEIN-A SEPHAROSE BEADS AND WAS OBSERVED IN IMMUNE-COMPLEX KINASE ASSAYS FOR PP60C-SRC

Using an immune complex kinase assay to measure pp60c-src kinase activ ity, we have identified a 56,000 M(r) protein (p56) from PC12 cell lys ates that co-purified with pp60c-src by strong association with protei n-A sepharose beads. The p56 protein was strongly phosphorylated on se rine but no tyrosine or threonine phosphorylation was evident. However , pp60c-src was strongly phosphorylated on tyrosine, weakly phosphoryl ated on serine with no observed threonine phosphorylation. P56 was not a proteolytic breakdown product of pp60c-src, since it was neither ty rosine phosphorylated nor was it recognized by anti-src antibody. P56 was also not recognised by other antibodies to 56kD signalling molecul es such as p56lck. The identity of p56 awaits further investigation bu t its appearance in immunoprecipitates of pp60c-src using protein-A se pharose beads is of interest but complicates the interpretation of res ults from immune complex kinase assays in PC12 cells.