Ham-2 corrects the class I antigen-processing defect in RMA-S cells

Abstract

The murine major histocompatibility complex (MHC) contains two genes (Ham-1 and Ham-2) that encode members of a super-family of ATP-dependent transport proteins. These genes are believed to mediate the transport of peptide antigen from the cytoplasm into the lumen of the endoplasmic reticulum for binding by MHC class I molecules. Evidence for such a function has come from the rescue of class I surface expression by a cloned copy of the human homologue of Ham-1, PSF-1, in a human cell line that is defective in antigen processing. A mutant murine cell line, RMA-S, has an identical antigen-processing-defective phenotype. Here we show that expression of a cloned copy of the Ham-2 gene in RMA-S cells results in recovery of the ability to process and present class I-restricted antigens to cytotoxic T lymphocytes, and in partial recovery of class I surface expression. Processing defects for classical (H-2 K and D) and non-classical (Qa1 and HMT) class I molecules are corrected by Ham-2. These data indicate that both MHC-linked transporter genes are probably required for class I antigen processing, and that the functional transporter in this pathway may consist of a Ham-1/Ham-2 heterodimer.

Functional expression and purification of the ABC transporter complex associated with antigen processing (TAP) in insect cells

“…Two lines of evidence support the idea that the TAPI/2 complex translocates antigenic peptides into the lumen of the ER for association with MHC class I molecules: (i) restoration of correct antigen presentation in defective mutant cell lines transfected with the corresponding human [21], mouse [22] or rat [23] tap genes, and (ii) peptide translocation assays using permeabilized cells [24,25] or microsomes prepared from mammalian cell lines [26]. In general, these assays take advantage of core glycosylation or binding of translocated radiolabelled peptides to MHC class I molecules in the ER or microsomal lumen, respectively, preventing rapid export out of the ER.…”

Defective iron homeostasis in beta 2-microglobulin knockout mice recapitulates hereditary hemochromatosis in man.

“…The TAP-1 and TAP-2 proteins are required for transport ofcytosolic peptides into the endoplasmatic reticulum for association with classical class I molecules (55)(56)(57). TAP-1 mutant mice are deficient in peptide transport, and consequently do not express classical class I molecules on the surface of their cells (18).…”

Misfolded major histocompatibility complex class I molecules accumulate in an expanded ER-Golgi intermediate compartment.

“…Ubiquitinylation and proteolysis by proteasomes have been proposed as major candidates (Michalek et al, 1993;Dick et al, 1994;Rock et al, 1994). The resultant peptides are then translocated into the lumen of the RER by the transporter associated with antigen presentation (TAP) (Powis et al, 1991;Spies and DeMurs, 1991;Attaya et al, 1992). TAP is a heterodimer composed of two subunits (TAP1 and TAP2) located in the ER and cis-Golgi , and is responsible for the translocation of peptides of particular size and sequence (Androlewicz et al, 1993;Neefjes et al, 1993;Shepherd et al, 1993;Heemels and Ploegh, 1995).…”