The multifaceted roles of the invariant chain CD74--More than just a chaperone.

The invariant chain (CD74) is well known for its essential role in antigen presentation by mediating assembly and subcellular trafficking of the MHCII complex. Beyond this, CD74 has also been implicated in a number of processes independent of MHCII. These include the regulation of endosomal trafficking, cell migration and cellular signalling as surface receptor of the pro-inflammatory cytokine macrophage migration inhibitory factor (MIF). In several forms of cancer, CD74 is up-regulated and associated with enhanced proliferation and metastatic potential. In this review, an overview of the diverse biological functions of the CD74 protein is provided with a particular focus on how these may be regulated. In particular, proteolysis of CD74 will be discussed as a central mechanism to control the actions of this important protein at different levels.