Invariant chain regulates endosomal fusion and maturation through the SNARE Vti1b

Abstract

30Invariant chain (Ii) is an important multifunctional player in the regulation of adaptive 31 immune responses and is responsible for several cellular functions related to MHCI and MHCII 32 antigen loading and antigen presentation. While regulating endosomal trafficking of MHCII 33 and other proteins that bind to Ii, this molecule is able to influence the endosomal pathway 34 delaying the maturation of endosomes to the late endosomal loading compartments. When 35 expressed in cells Ii is found to increase endosomal size, but the mechanisms for this is not 36 known. We used RNAi silencing to identify SNARE proteins controlling Ii induced increase 37 of endosomal size and delay of the endosomal pathway. Ii was found to interact with the 38 SNARE protein Vti1b. Vti1b localized at the contact sites of fusing Ii positive endosomes and 39 a tailless Ii was able to relocate Vti1b to the plasma membrane. Furthermore, silencing Vti1b, 40 abrogated the delay in endosomal maturation induced by Ii expression. In conclusion, Ii 41 interacts with Vti1b and this interaction is fundamental for Ii-mediated alteration of the 42 endosomal pathway. We propose that Ii, by interacting with SNAREs, in particular Vti1B in 43 the biosynthetic pathway of antigen presenting cells, is able to assemble SNARE directed 44 fusion partners in the early part of the endosomal pathway that lead to a slower endosomal 45 maturation for efficient antigen processing and antigen loading.Professional antigen presenting cells like dendritic cells and B cells express major 65 histocompatibility complex class II (MHCII) and associated molecules in the MHC region and 66 invariant chain (Ii, or CD74) located on a different chromosome (Genuardi and Saunders 1988). 67While MHCII is a polymorphic protein, Ii is a non-polymorphic type II transmembrane protein 68 that forms complexes by self-trimerization and binding to MHC class II (reviewed in 69 (Landsverk, Bakke, and Gregers 2009). Ii is mainly expressed in antigen presenting cells 70 (APCs) and has a prominent role sorting associated molecules to the endosomal pathway 71 (reviewed in (Schroder 2016)). Importantly, Ii, which contains two efficient leucine based 72 endosomal sorting signals, facilitates the exit of MHC class II from the endoplasmic reticulum 73 (ER) in antigen-presenting cells and mediates rapid transport of the MHCII/Ii complex to the 74 endosomal pathway where the antigen is efficiently loaded (Bremnes et al. 1994; Bakke and 75 Dobberstein 1990; Elliott et al. 1994; Bikoff et al. 1993; Neefjes and Ploegh 1992). In addition, 76 Ii delayed transport from Rab5 positive early endosomes (EE) to late Rab7a positive 77 endosomes (LE) (Gorvel et al. 1995). Ii also induced fusion of early endosomes creating 78 enlarged endosomes (Pieters, Bakke, and Dobberstein 1993; Romagnoli et al. 1993). These 79 enlarged endosomes were able to mature normally, however, with a prolonged residence time 80 in EE (presence of EEA1 and Rab5) before the endosomes switched to late, acidic Rab7a and 81 Lamp1 posit...