Mechanism of myosin II–mediated release of microparticles from endothelial cells activated by anti-β₂GPI antibodies. In this diagram, crosslinking of β₂GPI bound to a cell surface receptor (ie, annexin A2) leads to activation of intracellular signaling, including the TLR4–NF-κB pathway. ERK 1/2 and ROCK are also activated, though the pathways leading to activation of these kinases have not been clearly defined. We propose that ERK mediates activation of MLCK and that MLCK and ROCK mediate phosphorylation of RLC on T18/S19; activation by both MLCK and ROCK may be needed to achieve sufficient RLC phosphorylation to trigger subsequent events. Phosphorylated RLC causes the myosin heavy chain to unfold and bind actin, leading to submembranous actin-myosin assembly. Submembranous contractile forces promote budding of microparticles from the endothelial cell surface membrane. Y-27632, ML-7, and blebbistatin inhibit various steps in this pathway, impairing microparticle release. However, the expression of cell surface E-selectin is not impaired by these inhibitors.