[1] It delivers cytosolic peptides into the endoplasmic reticulum (ER), where they bind to nascent MHC class I molecules.
It has also been shown that undocking of the loaded MHC class I is linked to the transport cycle of TAP caused by signals from the TAP-1 subunit.
TAPs mediate the interaction of the messenger ribonucleoprotein particle (mRNP) and the nuclear pore complex (NPC).NXFs bear no resemblance to prototypical nuclear transport receptors of the importin – exportin (karyopherin) family and lack the characteristic Ran-binding domain found in all karyopherins.
The ATPase activity of TAP is highly dependent on the presence of the correct substrate, and peptide binding is prerequisite for ATP hydrolysis.
Peptides with low affinity for the MHC class I molecule are transported out of the ER by an efficient ATP-dependent export protein.