Major histocompatibility complex-linked transport proteins and antigen processing.

The major histocompatibility complexes of mice, rats and humans each contain a pair of related genes, Tap-1 and Tap-2, that encode members of a large superfamily of proteins having similar structure and function. The TAP-1 (previously called HAM1 in the mouse) and TAP-2 (HAM2) proteins each contain 6-8 predicted membrane-spanning alpha helices, and a cytoplasmic domain containing a putative ATP-binding site. Recent evidence suggests that a functional TAP-1/TAP-2 heterodimer is required for efficient presentation of antigens to CD8+ cytotoxic T cells. This heterodimer resides in the membrane of the endoplasmic reticulum (ER), and probably functions to transport peptides (produced in the cytoplasm) into the ER lumen for binding to MHC class I molecules.