Proteomic analyses reveal that immune integrins are major targets for proteolysis 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 down-regulating 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 down-regulate cell surface expression of the inflammatory cytokine receptor for interleukin-6 (IL6Ra). We used over-expression of these MARCH proteins in the M1 myeloid leukemia cell line and cell surface proteomic analyses to globally analyse other potential targets of these proteins. A large range of overlapping and a few unique cell surface proteins were identified most of which were down-regulated by MARCH expression. Prominent amongst these were several integrin complexes associated with immune cell homing, adhesion and migration. Integrin 41 (VLA4 or VCAM-1 receptor) was down-regulated only by MARCH2 and we showed that in MARCH2 knockout mice Integrin 4 was up-regulated specifically in mature B-lymphocytes and this was accompanied by decreased numbers of B-cells in the spleen.