We recently demonstrated that the mechanism of processing of an
HLA-A*0201-restricted
peptide epitope, Tyr(369)(D), derived from the
membrane protein tyrosinase, involves retrotranslocation of glycosylated molecules from the endoplasmic reticulum to the cytosol, removal of an N-linked
carbohydrate from Asn(371) by
peptide N-glycanase, proteolysis by the
proteasome and other
proteases, and retransport of the resulting
peptides into the endoplasmic reticulum for association with
HLA-A*0201.
Carbohydrate removal results in deamidation of Asn(371) to
aspartic acid. The
asparagine-containing homolog of this
peptide, Tyr(369)(N), is not presented by
tyrosinase-expressing cells, and this has been presumed to be due to quantitative glycosylation of Asn(371). Although examining cytosolic intermediates that accumulated in human
melanoma cells treated with
proteasome inhibitors, we were surprised to find both molecules that had been deglycosylated by
peptide N-glycanase and a large number of molecules that had not been previously glycosylated. The failure of Tyr(369)(N) to be processed and presented from these latter molecules may be partially due to a process of deamidation independent of glycosylation. However, we also established that proteasomes degrade
tyrosinase molecules that are still glycosylated, giving rise to a set of discrete intermediates that are not observed when unglycosylated molecules are degraded. We propose that Tyr(369)(N) fails to be presented because unglycosylated
tyrosinase is degraded rapidly and relatively nonselectively. In contrast, glycosylation alters the selectivity of
tyrosinase processing by the
proteasome, enhancing the production or survival of Tyr(369)(D).