Expression of p60fyn in human platelets.

Platelets possess high levels of tyrosine protein kinase activity which has been associated, in part, with the abundant expression of pp60c-src. We report here that in addition to pp60c-src at least one other member of the src family of tyrosine protein kinases, p60fyn, is expressed in platelets. The abundance of p60fyn was estimated to be approximately 20- to 40-fold higher in platelets than in human fibroblasts. In platelets the abundance of p60fyn was determined to be approximately 5- to 10-fold lower than the abundance of pp60c-src. Thrombin-mediated activation of human platelets was found to rapidly elevate the level of detectable phosphotyrosine containing proteins. However, thrombin treatment of platelets did not result in significant alterations in either the abundance or activity of pp60c-src or p60fyn. These observations demonstrate that at least two members of the src family of tyrosine protein kinases (pp60c-src and p60fyn) are expressed in human platelets, but leave unresolved the question of whether these protein kinases play a role in platelet signal transduction events.