The intracellular transport route taken by newly synthesized class II molecules to the cell surface diverges from the constitutive secretory pathway used for class I export. Thus, class II molecules selectively capture antigenic peptides inside endocytic vesicles. The Ii chain contains endosomal targeting signal(s) located near the N-terminus, and has been shown to direct transport of immature class II dimers to peripheral acidic compartments. Selective Ii chain degradation subsequently results in class II peptide occupancy. As far as we know, under physiological conditions the vast majority of mature class II molecules expressed at the cell surface are devoid of Ii chain and are instead tightly occupied by an appropriate peptide ligand.

The Ii chain associates with MHC class II hetero-dimers to form nine subunit complexes shortly after chain synthesis in the endoplasmic reticulum (ER). As a general rule, assembly of allelically matched a(3 pairs does not require Ii chain coexpression, but in the exceptional case of AabA|3b molecules, Ii chain expression has been shown to facilitate production or maintenance of a(3 dimers. In the absence of Ii chain, empty class II molecules tend to aggregate, a property reversed upon occupancy of the peptide-binding site. Thus, the Ii chain serves as the functional equivalent of peptide for class II during early folding of the nascent chains, and protects the otherwise exposed, empty groove from being recognized by ER resident proteins. As for other incompletely or partially assembled oligomeric surface receptors, class II in the absence of Ii chain associates with molecular chaperones such as BiP and calnexin, thought to be responsible for quality control in the ER.

The Ii chain facilitates transport of correctly folded class II a(3 dimers through the Golgi apparatus en route to the cell surface. Under physiological conditions, association with Ii chain insures that newly formed a|3 heterodimers are quantitatively exported to endocytic vesicles, thus promoting selective interactions with peptides derived from internalized antigens.

Several closely related Ii chain polypeptides arise from a single Ii chain gene. Comparisons of the Ii chain gene products expressed in humans, mice and rats demonstrate a high degree of evolutionary conservation of structure and function. In humans, the 16 amino acid extension encoded by an alternative in-frame AUG start codon, has been found to contain an ER retention signal. The cytoplasmic domain

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