The role of signaling adaptor GIT1 in the regulation of cell migration

GIT1 is a multi-domain protein that functions as an integrator of several migration-related signaling pathways. We demonstrated that GIT1 cycles between at least three distinct subcellular compartments, including adhesion-like structures, the leading edge, and cytoplasmic complexes. GIT1 is targeted to each of these compartments by separate domains. GIT1 cytoplasmic complexes are multi-protein supramolecular structures rather than components of a membranous compartment. These complexes are motile and can move toward forming adhesions at the cell front and away from the ones disassembling in retracting areas of the cell, suggesting that GIT1 can traffic signaling molecules to specific locations in the cell. Expression of GIT1 increases the rate of migration, and the size and the number of protrusions. GIT1 also targets constitutively activated PAK to adhesions and the leading edge through its interaction with paxillin. We utilized mass-spectrometry to identify 19 phosphorylation sites on GIT1, which include 10 serines, 7 tyrosines and 2 threonines. Serine 709 is located in the paxillin binding domain and is an in vitro target for PAK. This site is required for proper localization and function of GIT1. GIT1 enhances formation and disassembly of adhesions in protruding areas of the cell and this ability depends on its ARF-GAP activity. Additional functions of GIT1 and mechanisms of its regulation may be revealed through some of its novel binding partners which we identified in another mass-spectrometry based study. Taken together these data demonstrate that GIT1 is an important regulator of migration and functions as a key component of a motile, multi-molecular complex. The effects of GIT1 on migration are mediated via several signaling pathways and regulated by phosphorylation.