Alternative mechanisms of regulation. (A) Established and novel mechanisms of regulating SFK activity in platelets. (B) Unclamping and activation of SFKs by disruption of the intramolecular interactions by a proline-rich (PxxP)/phosphotyrosine-containing protein. Disruption of either the SH3-proline-rich linker or the SH2 C-terminal phosphotyrosine intramolecular interactions facilitates SFK activation, irrespective of whether the C-terminal inhibitory tyrosine residue is phosphorylated. Ser/Thr PPs, protein-phosphatases.