Proteins inside the cell cytosol may be degraded by a Low Molecular Weight Protease complex (LMP or Proteasome) and moved from the cytoplasm to the rough endoplasmic reticulum (RER) by the TAP proteins. These are members of the ATP-binding cassette (ABC) superfamily which includes genes whose products are transmembrane proteins involved in energy-dependent transport of a wide spectrum of substrates across membranes.
The two MHC-linked genes, TAP1 [embl: X57522] and TAP2 [embl: M84748] (for Transporter Associated with antigen Processing), are required for normal presentation of intracellular antigens to T cells. These genes encode the polypeptides that form a heteromeric "peptide pump". The TAP1 (also known as RING4 or PSF1) and TAP2 (also known as RING11 or PSF2) genes possess an ATP binding cassette and 6 to 8 transmembrane helical segments. They are responsible for peptides selection and movement across the ER membrane to the binding site of MHC class I molecules.
Uebel S, 1997
Recognition principle of the TAP transporter disclosed by combinatorial peptide libraries.
Proc Natl Acad Sci U S A 94(17), 8976-8981 (1997)
Elliott T, 1997
How does TAP associate with MHC class I molecules?
Immunol Today 18(8), 375-379 (1997)
Ortmann B, 1994
MHC class I/beta 2-microglobulin complexes associate with TAP transporters before peptide binding.
Nature 368(6474), 864-867 (1994)
TRANSPORTER 1, ABC; TAP1