Peptide Binding Inhibits Aggregation of Soluble MHC Class II in Solution

Abstract

SummaryAf nity-puri ed major histocompatability complex (MHC) class II molecules are known to bind antigenic peptide in vitro. This peptide-bound MHC class II is known to undergo a change in structure upon stable binding of antigenic peptide. Previous results from our, and other laboratories, have suggested a relationship between MHC class II structure and peptide association that enables class II to enter into a stable conformation upon peptide binding. INTRODUCTIONMajor histocompatibility complex (MHC) 2 class II antigens are heterodimeric transmembrane cell surface glycoproteins that bind antigenic peptides and display them on the cell surface (1, 2). These complexes of MHC class II with antigenic peptide are then speci cally recognized by T cell receptors on the surface of T cells, leading to the MHC-restricted T cell activation (3, 4).MHC class II molecules are heterodimers with a molecular mass of 60 kDa, consisting of a 32-kDa a chain and 28-kDa b chain. The a and b chains are structurally homologous to one another, both being type I integral membrane proteins with small cytoplasmic domain and large extracellular domains. It has been documented that MHC class II proteins isolated from B lymphoblastoid cells is occupied with a wide spectrum of peptides (5-7). Several studies from different laboratories have shown that puri ed MHC class II molecules isolated from lymphoblastoid cells are capable of binding antigenic peptide in vitro (8-11) or in vivo (5-7, 12). In vitro, the percent occupancy of MHC class II molecules with antigenic peptides varies significantly. However, previous results from our laboratory showed that in vitro af nity-puri ed HLA-DR2 molecules could be fully loaded with high-af nity antigenic peptide under optimized conditions (13).Several studies with MHC class II molecules have been performed in vitro to understand a relationship between MHC class II structure and peptide binding. For example, class II molecules solubilized from cell surfaces with detergent show improved stability upon peptide binding in sodium dodecyl sulfate (SDS) (14). Similarly, baculovirus-expressed soluble human DR1 class