The mouse MHC T region encoded class I-b molecules, TL and Qa- 1, are the focus of this new application. Functions for class I-b proteins have not been fully elucidated, but growing evidence indicates a role in peptide binding and antigen presentation similar to that of the class I-a molecules. In addition, these proteins have distinctive patterns of expression and structural features which support the concept that they play specialized and important roles in immune regulation. We propose to define the peptides bound to and antigen presentation functions of Qa-1 and TL and to elucidate the molecular mechanisms that regulate their expression. Our first Aim is to isolate and sequence peptides released from the Qa-1 and TL molecules and to examine binding of these peptides to soluble forms of these molecules using surface plasmon resonance. These studies will answer questions regarding the types of peptides these proteins bind, whether there are specialized requirements for binding, and whether TL expressed independently of TAP1,2 can bind peptides or is stable in the absence of peptide. The second AIM will extend the above studies and define peptide presentation functions of class 1-b molecules using CTLs. Roles for Qa-2 in presentation of a RAG-1 peptide, Qa-1 in presentation of TCR Vbeta peptides and the H-2Ld signal peptide (Ld3-11), and TL in TAP-independent versus dependent peptide presentation will be determined. Aim three will define the novel protein Qsm that heterodimerizes with the Qa-1 molecule and determine if it is encoded by the H-2K2b gene or by a previously undescribed, perhaps class I-like gene. We will address mechanisms that govern Qa-1/Ld heterodimerization; these include roles of the domains of Ld, bound peptides, oligosaccharides, and Beta2m. Our fourth Aim will answer questions relating to antigen presentation function and heavy chain processing of Qa-1. We will define the mechanism of intracellular instability of Qa-1 by examining the involvement of 1)specific heavy chain domains; 2) an unusual and potentially uncoupled cysteine residue at position 114 in the heavy chain; 3) Beta2m; and 4)peptide supply/availability. We are confident these proposed investigations will answer fundamental questions relating to immune function of class 1-b molecules and elucidate molecule mechanisms that control their unusual expression. Insight into structure-function relationships and the biological roles of class I molecules, in general, will emerge from these studies.