Subcellular localization of the melanoma-associated protein Melan-A(MART-1) influences the processing of its HLA-A2-restricted epitope

The peptide derived from the melanoma-associated protein Melan-A (Melan-A(2 6-35)/HLA-A2) is an attractive candidate for tumor immunotherapy but little is known about the intracellular processing of this antigen. Here we show that Melan-A is a single-pass membrane protein with an NH2 terminus exposed to the lumen of the exocytic compartment. In transfected melanoma cells, M elan-A accumulates in the Golgi region. Inversion of the membrane topology leads to the retention of Melan-A in the endoplasmic reticulum. Most striki ngly, melanoma cells expressing this form of Melan-A are more effectively r ecognized by specific CTL than those expressing either Melan-A in its nativ e membrane orientation or Melan-A artificially localized in the cytosol. Ou r data are compatible with the notion that proteins retained in the endopla smic reticulum are more efficiently degraded and produce more antigenic pep tides.