Identification of the tyrosine protein kinase from LSTRA cells by use of site-specific antibodies.

The lymphoma cell line LSTRA contains an elevated level of tyrosine protein kinase activity. It has been suggested that this elevated level of activity is due to the presence of a phosphoprotein with a molecular weight of 56,000 (pp56, formerly referred to as a 58,000-dalton protein). This paper describes the preparation of antibodies against pp56 through the use of a synthetic peptide that contains the sequence around the site of tyrosine phosphorylation in pp56, which is identical to the phosphorylation site in pp60src. These antipeptide antibodies specifically immunoprecipitated 32P-labeled pp56 from detergent extracts of LSTRA cells. In immunoblotting experiments, pp56 was the major antigen detected in the particulate fraction from LSTRA cells by the antipeptide antibodies. The antibodies were also used to show that the level of pp56 is greatly elevated in LSTRA cells. Incubation of the detergent extract of the particulate fraction from LSTRA cells with the antipeptide antibodies resulted in inhibition of most of the LSTRA cell tyrosine protein kinase activity. These results indicate that pp56 is the tyrosine protein kinase whose activity is elevated in LSTRA cells. This enzyme may be a member of the large family of protein kinases involved in the regulation of cell growth.