The Association of Cargo Receptor Bap31 with MHC I is Regulated by Bap29

Abstract

The proper cell surface presentation of major histocompatibility complex class I (MHC I) molecules loaded with high‐affinity peptide is critical for T cell recognition of pathogens. MHC I is assembled with a peptide in the endoplasmic reticulum (ER). We present evidence of a novel post‐ER quality control mechanism that ensures only high‐affinity peptide‐loaded MHC I (pMHC I) molecules reach the cell surface. The cargo receptor Bap31 aids in the ER export of pMHC I to the ER‐Golgi intermediate compartment (ERGIC). Bap31 then recycles low‐affinity pMHC I molecules back the ER, while high‐affinity pMHC I molecules continue to traffic to the cell surface. We use fluorescence resonance energy transfer (FRET) and quantitative fluorescence imaging in HeLa cells to show clustering between Bap31 and MHC I. We also show that anterograde traffic of both Bap31 and MHC I to the ERGIC is increased upon a short pulse of high‐affinity peptide, which creates a bolus of MHC I molecules ready for ER export. An intermediate increase is shown when cells are fed with a low‐affinity peptide. Overexpression of Bap29 in cells inhibits Bap31 traffic to the ERGIC and decreases MHC I surface expression. Bap29 is a homolog of Bap31 and has been shown to form heterodimers with Bap31. Unlike Bap31, Bap29 is only found to localize in the ER. Whereas, overexpression of Bap31 increases MHC I cell surface expression and stability. Thus, Bap31 is directly involved in MHC I ER export and this function is regulated by Bap29.