Csk and CD148 reciprocally regulate platelet SFKs. (Ai) Representative electropherograms of capillary-based immunoassays on platelet lysates with the indicated antibodies and the quantification of peak areas (n = 6 mice/genotype. (Aii) Representative data from panel Ai displayed as blots. See also supplemental Figure 8. (B) Model of SFK regulation in platelets. In WT platelets, SFKs are constrained in an inactive conformation by Csk, which phosphorylates the C-terminal inhibitory tyrosine residue. SFKs can be activated by dephosphorylation of the inhibitory residue by CD148. SFKs trans-autophosphorylate each other at the activation-loop tyrosine residue and become fully active. CD148 can also dephosphorylate the activation-loop tyrosine leading to a decrease in SFK activity. In Csk KO platelets, CD148 dominates, resulting in the loss of inhibitory phosphorylation and a net increase in SFK activity. In CD148 KO platelets, Csk dominates, resulting in increased inhibitory phosphorylation and markedly decreased activation-loop phosphorylation. In DKO platelets, the absence of both Csk and CD148 leads to a dramatic increase in SFK activity. The differential phosphorylation of SFKs in Csk KO and DKO platelets supports the hypothesis of CD148 dephosphorylating both the activation-loop and the C-terminal inhibitory tyrosine residues. Professional illustration by Patrick Lane, ScEYEnce Studios. (C) Platelet lysates were blotted for the indicated proteins. *P < .05, **P < .01, ***P < .001; repeated measures 1-way ANOVA with Tukey’s test; data represent mean ± SEM.