Regulation of Ped gene expression by TAP protein

The Ped (Preimplantation embryo development) gene regulates fast or slow cleavage of preimplantation mouse embryos and their subsequent survival. The protein product of the Ped gene is the major histocompatibility complex (MHC) class Ib protein Qa-2. MHC class I expression on the cell surface requires the assembly within the endoplasmic reticulum (ER) of an α heavy chain, a β 2 microglobulin light chain, and a small peptide. Small peptides are primarily produced in the cytosol by the ubiquitin–proteasome pathway and then are transported into the ER by the transporter associated with antigen processing (TAP) protein. However, some peptides can bind to MHC class I heavy chains in a TAP-independent manner. In this study, we assessed whether TAP protein regulates Qa-2 expression on the cell surface of preimplantation mouse embryos thereby influencing the PED phenotype of the embryos. We chose Tap1 knockout mice and their control mice (B6.129) as our experimental system. We analyzed Qa-2 mRNA expression by RT–PCR, total Qa-2 protein expression by Western blotting, and cell surface Qa-2 protein expression by Immuno-PCR in preimplantation embryos of both Tap1 knockout mice and control mice. Then we determined the PED phenotype of both Tap1 knockout mouse embryos and control mouse embryos. The results showed that Qa-2 expression on the cell surface of preimplantation embryos is dependent on TAP protein, and that Qa-2 expression on the cell surface is required for expression of the fast PED phenotype.