Characteristics of ATP-dependent peptide transport in isolated microsomes

Abstract

This report examines the transport properties and specificity of ATP-dependent peptide transport by the murine transporter for Ag presentation (TAP) complex in isolated microsome preparations from H-2d haplotype mice. The murine TAP complex has a Km of 661 nM and a maximum velocity of 2.9 fmol/min.micrograms microsome protein for a modified peptide corresponding to a defined MHC class I binding epitope from influenza nucleoprotein recognized by CD8+ CTL in association with the Kd molecule. This high Km value for peptide transport suggests that the rate and efficiency of peptide transport of the TAP complex are influenced by the concentration of processed peptides derived from self and foreign proteins in the cell cytoplasm. Furthermore, these findings imply that competition among peptides for TAP-dependent transport is unlikely to be an important factor in determining the immunodominance of certain peptide epitopes within a foreign protein recognized by CD8+ T lymphocytes. We also examined the specificity of TAP transport for peptides containing bona fide murine MHC class I binding epitopes and provide evidence that certain flanking residues can affect the efficiency of peptide epitope transport by the TAP complex.