Proteomic analyses reveal that immune integrins are major targets for regulation by Membrane-Associated Ring-CH (MARCH) proteins MARCH2, 3, 4 and 9

MARCH proteins are membrane-associated Ring-CH E3 ubiquitin ligases that dampen immune responses by downregulating cell surface expression of major histocompatibility complexes I and II as well as immune co-stimulatory receptors. We recently showed that MARCH2,3,4 and 9 also downregulate cell surface expression of the inflammatory cytokine receptor for interleukin-6 (IL6Rα). Here we use over-expression of these MARCH proteins in the M1 myeloid leukaemia cell line and cell surface proteomic analyses to globally analyse other potential targets of these proteins. A large range of cell surface proteins regulated by more than one MARCH protein in addition to several MARCH protein-specific cell surface targets were identified most of which were downregulated by MARCH expression. Prominent among these were several integrin complexes associated with immune cell homing, adhesion and migration. Integrin α4β1 (VLA4 or VCAM-1 receptor) was downregulated only by MARCH2 and we showed that in MARCH2 knockout mice, Integrin α4 was upregulated specifically in mature B-lymphocytes and this was accompanied by decreased numbers of B-cells in the spleen.