INTRACELLULAR LOCATION, COMPLEX-FORMATION, AND FUNCTION OF THE TRANSPORTER ASSOCIATED WITH ANTIGEN-PROCESSING IN YEAST

Peptide transport across the membrane of the endoplasmic reticulum (ER ) gains increasing importance in view of its potential function in sel ective protein degradation and antigen processing. An example for pept ide transport in the ER is the transporter associated with antigen pro cessing (TAP), which supplies peptides for the formation of major-hist ocompatibility-complex class-I complexes. Here, we have expressed huma n TAP1 and TAP2 in the yeast Saccharomyces cerevisiae. Expression of b oth genes resulted in the formation of a stable TAP heterodimer that w as localized mainly in the ER. Although a minor fraction of TAP is fou nd in the plasma membrane, TAP is unable to restore a-factor secretion in a mutant cell line that lacks the yeast mating-factor transporter Ste6. Nevertheless, in vitro studies with microsomal vesicles demonstr ated that the TAP complex is fully functional in the ER membrane in te rms of selective peptide binding, ATP-dependent transport, and specifi c inhibition by the viral protein of herpes simplex virus ICP47. This offers opportunities for topological, structural and mechanistic studi es as well as genetic screenings for TAP functionality.