4N1K causes an early increase in tyrosine phosphorylation of platelet proteins. Washed platelets adherent to collagen for the indicated times in the presence or absence of 4N1K (as in Fig 2) were lysed in RIPA buffer. Platelets were preincubated without (left) or with PGE1 (right) for 15 minutes. Lysates were subjected to immuno-precipitation with PY20 (antiphosphotyrosine) and 4G10 (antiphosphotyrosine) immunoblot analysis.