Newly synthesized class II molecules of the major histocompatibility complex must be transported to endosomal compartments where antigens are processed for presentation to class II-restricted T cells. The invariant chain (Ii), which assembles with newly synthesized class II alpha- and beta-chains in the endoplasmic reticulum, carries one or more targeting signals for transport to endosomal compartments where Ii dissociates from alpha beta Ii complexes. Here we show that the transport route of alpha beta Ii complexes is regulated selectively by two forms of Ii (p33 and p35) that are generated by the use of alternative translation initiation sites. Using a novel quantitative surface arrival assay based on labeling with [6-3H]-D-galactose combined with biochemical modification at the cell surface with neuraminidase, we demonstrate that newly synthesized alpha beta Ii molecules containing the Ii-p33 isoform can be detected on the cell surface shortly after passage through the Golgi apparatus/trans-Golgi network. A substantial amount of these alpha beta Ii complexes are targeted to early endosomes either directly from the trans-Golgi network or after internalization from the cell surface before their delivery to antigen processing compartments. The fraction of alpha beta Ii complexes containing the p35 isoform of Ii with a longer cytosolic domain was not detected at the cell surface as determined by iodination of intact cells and the lack of susceptibility to neuraminidase trimming on ice. However, treatment with neuraminidase at 37 degrees C did reveal that some of the alpha beta Ii-p35 complexes traversed early endosomes. These results demonstrate that a fraction of newly synthesized class II molecules arrive at the cell surface as alpha beta Ii complexes before delivery to antigen processing compartments and that class II alpha beta Ii complexes associated with the two isoforms of Ii are sorted to these compartments by different transport routes.

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