Meta-Biol

• Pathways

In addition to the membrane remodeling for pseudopod extension, particle internalization requires a contractility force pulling the forming phagosome into the cytoplasm. Myosin motor proteins are the actin-binding proteins, with ATPase activity move along actin fibers, and produce the driving force for phagosome formation and transport. Several myosin motors including myosins IC, II, V, IXb are involved in FCGR-mediated phagocytosis as force generators and actin-based transport motors (Swanson et al. 1999). Nonmuscle myosin II, is a motor protein known to generate intracellular contractile forces and tension by associating with F-actin. It has been observed to localize around forming phagosomes and suggested a role in phagocytic-cup squeezing during FCGR-mediated phagocytosis. Each myosin II motor protein exists as a complex consisting of two copies each of myosin II heavy chain (MHC), essential light chains (ELC), and myosin regulatory light chain (MRLC). Selective inhibition of myosin II by ML-7, a myosin light-chain kinase (MLCK) inhibitor, prevents phagocytic cup closure, but not pseudopod extension for the formation of phagocytic cups in FCGR-mediated phagocytosis (Grooves et al. 2008, Araki 2006). Tight ring of actin filaments within the elongating pseudopodia squeezes the deformable particles. In the classical zipper model for phagocytosis, the pseudopod extends over the IgG-coated particles, in which FCGRs in the phagocyte plasma membrane interact sequentially with Fc portions of IgG molecules zippering the membrane along the particle. This sequential IgG-FCGR binding might not occur by itself, but requires forced zipper closure, where myosin-II contractile activity may promote the binding between the FCGR and its ligands, to facilitate the efficient extension and subsequent closure of phagocytic cups (Araki 2006, ). Myosin IC mediates the purse-string-like contraction that closes phagosomes. Myosin-V has been implicated in membrane trafficking events (Swanson et al. 1999).