Active Syk is enriched in the ubiquitinated fraction. (A) An immunoblot of ubiquitinated Syk probed with an antibody to total Syk (lane 1) is compared with a blot of ubiquitinated Syk probed with a phosphospecific (pY525/pY526) antibody (lane 2). (B) The FK2 fraction of convulxin-activated platelets (○ and —) and the Syk fraction obtained after the majority of the ubiquitinated form was removed (▪ and —) were assayed for their kinase activity as in Figure 5. The relative protein mass in each fraction was assessed from immunoblots using the phosphospecific antibody for Syk residues pY525/pY526. Error bars indicate standard error of three determinations.