Rab13 acts downstream of the kinase Mst1 to deliver the integrin LFA-1 to the cell surface for lymphocyte trafficking.

In lymphocytes, the kinase Mst1 is required for the proper organization of integrins in the plasma membrane at the leading edge of migrating cells, which is critical for lymphocyte trafficking. We found a functional link between the small G protein Rab13 and Mst1 in lymphocyte adhesion and migration. In response to stimulation of T lymphocytes with chemokine, Mst1 promoted phosphorylation of the guanine nucleotide exchange factor DENND1C (differentially expressed in normal and neoplastic cells domain 1C), which activated Rab13. Active Rab13 associated with Mst1 to facilitate the delivery of the integrin LFA-1 (lymphocyte function-associated antigen 1) to the leading edge of lymphocytes. Delivery of LFA-1 involved the recruitment of myosin Va along actin filaments, which extended as a result of the localization of the actin regulatory protein VASP to the cell periphery through phosphorylation of VASP at Ser(157) by Mst1. Inhibition of Rab13 function reduced the adhesion and migration of lymphocytes on ICAM-1 (intercellular adhesion molecule-1), the ligand for LFA-1, and inhibited the formation of a ring-like arrangement of LFA-1 at the contact sites between T cells and antigen-presenting cells. The lymphoid tissues of Rab13-deficient mice had reduced numbers of lymphocytes because of the defective trafficking capability of these cells. These results suggest that Rab13 acts with Mst1 to regulate the spatial distribution of LFA-1 and the motility and trafficking of lymphocytes.