Hypothetical model for regulation of PECAM-1 tyrosine phosphorylation and inhibitory function by PECAM-1 cytoplasmic domain interactions with the plasma membrane. (A) Schematic diagram representing membrane proximity of amino acids within the human PECAM-1 cytoplasmic domain. Amino acids are identified with the single-letter code, and numbering is based on the sequence of the mature human protein. ITIM tyrosine residues are shown in yellow, and the serine residues that are susceptible to inducible (S₇₀₂) and constitutive (S₇₀₇) phosphorylation in platelets are shown in orange. The juxtamembrane cytoplasmic cysteine residue that is susceptible to palmitoylation is shown in red. A segment of the PECAM-1 cytoplasmic domain that spans amino acid residues T₆₈₂-S₇₀₂ forms an amphipathic helix that participates in plasma membrane interactions. Important features of the membrane-interacting segment are that it (1) is bordered by the inducibly phosphorylated serine residue (S₇₀₂), (2) encompasses the C-terminal ITIM tyrosine residue (Y₆₈₆), and (3) excludes the N-terminal tyrosine residue (Y₆₆₃). (B) Model for regulation of PECAM-1 cytoplasmic domain phosphorylation by reversible plasma membrane association. (i) In resting platelets, the PECAM-1 cytoplasmic domain, which is constitutively phosphorylated on S₇₀₇, binds to the plasma membrane via interactions between hydrophobic amino acids and neutral phospholipids (gray) and between positively charged amino acids and negatively charged phospholipids (black). (ii) Activation of a serine/threonine (S/T) kinase in activated platelets results in phosphorylation of S₇₀₂, which initiates dissociation of the membrane-interacting segment from the plasma membrane and exposes the C-terminal ITIM tyrosine residue (Y₆₈₆). (iii) Activation of a tyrosine kinase in aggregated platelets results in phosphorylation of Y₆₈₆, which enables subsequent phosphorylation of the N-terminal ITIM tyrosine residue (Y₆₆₃). Phosphorylation of both ITIMs supports SHP-2 binding and PECAM-1 inhibitory function.