ABSTRACT
The TAP complex transports peptides from the cytosol into the lumen of the endoplasmic reticulum for presentation by major histocompatibility complex class I molecules. A limited degree of sequence polymorphism has been observed for the mouse TAP1 and TAP2 genes by restriction fragment length polymorphism and sequence analysis. However, functional polymorphism of the TAP transporter has thus far been observed for the rat only. Here we examine the effect of TAP polymorphism on ATP dependency and peptide specificity of TAP-mediated peptide transport and show that, in the mouse, polymorphism in TAP genes does not measurably alter the function of their gene products. We conclude that TAP polymorphism is unlikely to contribute to the development of autoimmune diseases and that, in the mouse, the specificity of the TAP transporter is matched to that of the F pocket of the class I molecules for which it provides the peptide substrates.
Subject(s)
ATP-Binding Cassette Transporters/metabolism , H-2 Antigens/genetics , Mice, Inbred NOD/immunology , Peptides/metabolism , ATP Binding Cassette Transporter, Subfamily B, Member 2 , ATP Binding Cassette Transporter, Subfamily B, Member 3 , Adenosine Triphosphate/metabolism , Alleles , Amino Acid Sequence , Animals , Biological Transport, Active , Endoplasmic Reticulum/metabolism , Haplotypes , Mice , Mice, Inbred Strains , Molecular Sequence Data , Peptides/chemistry , Peptides/immunology , Polymorphism, Genetic , beta 2-Microglobulin/metabolismABSTRACT
The transporter associated with antigen processing (TAP) delivers peptides to the lumen of the endoplasmic reticulum in an adenosine triphosphate (ATP) dependent fashion for presentation by major histocompatibility complex class I molecules. We show that the mouse TAP translocator (H-2b haplotype) selects peptides based on a minimal size of nine residues, and on the presence of a hydrophobic COOH-terminal amino acid. The preponderance of COOH-terminal hydrophobic amino acids in peptides capable of binding to mouse class I molecules thus fits remarkably well with the specificity of the TAP translocator. In addition to transport in the lumenal direction, efflux of peptide in the cytosolic direction is observed in an ATP- and temperature-dependent manner. By maintaining a low peptide concentration at the site of class I assembly, this efflux mechanism may ensure that class I molecules are loaded preferentially with high affinity peptides.
