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Invariant chain regulates endosomal fusion and maturation through an interaction with the SNARE Vti1b.

The Invariant chain (Ii, CD74) is a multifunctional regulator of adaptive immune responses and responsible for sorting MHC-I, MHC-II and other Ii-associated molecules to a specific endosomal pathway. When Ii is expressed, endosomal maturation and proteolytic degradation of proteins are delayed and in non-antigen presenting cells the endosomal size increase, but he molecular mechanisms are not known. We identified that a SNARE, Vti1b, is essential for regulating these Ii induced effects. Vti1b binds to Ii and Vti1b is localized at the contact sites of fusing Ii positive endosomes. Furthermore, a tailless Ii that is not internalized from the plasma membrane relocates Vti1b to the plasma membrane. KO of Ii in an antigen presenting cell line was found to speed up endosomal maturation and silencing of Vti1b inhibits the Ii induced maturation delay. Our results suggest that Ii, by interacting with the SNARE Vti1b in antigen presenting cells, direct specific Ii associated SNARE mediated fusion in the early part of the endosomal pathway that lead to a slower endosomal maturation for efficient antigen processing and MHC antigen loading.