Transient surface delivery of invariant chain-MHC II complexes via endosomes: a quantitative study

Newly synthesized major histocompatibility complex class II needs to be directed to late endocytic compartments to combine with peptide antigens. Efficient transport requires complexes of major histocompatibility complex class II and invariant chain (alphabetali). Since such complexes have been detected on the plasma membrane in human cells, this compartment was proposed as the primary destination for alphabetali exiting the trans-Golgi network. Here, I have used density gradient electrophoresis and selective biotinylation to investigate the trafficking route of alphabetali quantitatively. Density gradient electrophoresis analysis showed that alphabetali was transported from the trans-Golgi network to endosomes at similar to1.7% min(-1). Surface delivery of alphabetali was delayed relative to endosome transport by similar to10min and showed slower kinetics (similar to0.4% min(-1)), suggesting that alphabetali reached the plasma membrane only after arrival in endosomes. A biotinylation assay revealed that 20-40% of endosomal alphabetali was delivered to the plasma membrane at steady state, suggesting that surface alphabetali was entirely derived from endosomes. Surface alphabetali was rapidly re-internalized and either returned to the cell surface or accessed degradative compartments. Peptide loading commenced similar to30min after delivery to endosomes. Thus alphabetali directly traffics from trans-Golgi network to endosomes and enters an endosome-plasma membrane 'carousel' until transport to peptide-loading compartments ensues.