CD74

HLA class II histocompatibility antigen gamma chain also known as HLA-DR antigens-associated invariant chain or CD74 (Cluster of Differentiation 74), is a is_associated_with::protein that in humans is encoded by the CD74 is_associated_with::gene. The invariant chain (Abbreviated Ii) is a is_associated_with::polypeptide involved in the formation and transport of is_associated_with::MHC class II protein. The cell surface form of the invariant chain is known as CD74.

Function
The nascent MHC class II protein in the rough ER has its peptide-binding cleft blocked by the invariant chain (Ii; a trimer) to prevent it from binding cellular peptides or peptides from the endogenous pathway. The invariant chain also facilitates MHC class II's export from the ER in a vesicle. The signal for endosomal targeting resides in the cytoplasmic tail of the invariant chain. This fuses with a late is_associated_with::endosome containing the endocytosed, degraded proteins. It is then cleaved by is_associated_with::cathepsin S (is_associated_with::cathepsin L in cortical thymic epithelial cells), leaving only a small fragment called CLIP which blocks peptide binding until is_associated_with::HLA-DM binds to MHC II, releasing CLIP and allowing other peptides to bind. The stable MHC class-II is then presented on the cell surface.

Possible interactions
In limited cases, CD74 might interact with is_associated_with::Macrophage migration inhibitory factor.