Bourgeois-Daigneault MC, Thibodeau J

MARCH1, a member of the membrane-associated RING-CH family of E3 ubiquitin ligases, regulates antigen presentation by down-regulating the cell surface expression of MHC class II and CD86 molecules. MARCH1 is a transmembrane protein that exposes both its N- and C-terminus to the cytoplasm. Here, we have conducted a structure-function analysis of its two cytoplasmic tails to gain insights into the trafficking of MARCH1 in the endocytic pathway. Fusion of the N-terminal portion of MARCH1 to a type II transmembrane reporter molecule revealed that this cytoplasmic tail contains endosomal sorting motifs. The C-terminal domain also appears to contain intracellular sorting signals as it reduced surface expression of a type I transmembrane reporter molecule. Mutation of the two putative C-terminal tyrosine-based sorting signals did not affect the activity of human MARCH1. However, it reduced its incorporation into exosomes. Moreover, site-directed mutagenesis pointed to a functional C-terminal (221)VQNC(224) sequence that affects the spatial organization of the two cytoplasmic regions. This motif is also found in other RING-type E3 ubiquitin ligases, such as parkin. Altogether, these findings highlight the complex regulation of MARCH1 trafficking in the endocytic pathway as well as the intricate interactions between its cytoplasmic tails.

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