This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Insertion of tail-anchored (TA) membrane proteins into the endoplasmic reticulum (ER) is mediated by the Transmembrane Domain Recognition Complex (TRC). The goal of our work is to understand the molecular basis of TA membrane protein targeting by TRC. The central component of TRC is a 40 kDa ATPase called TRC40 that functions as a dimer. We recently solved high resolution crystal structures of TRC40 in the absence and presence of nucleotide, and these reveal a large conformational change from an open- to a closed-dimer. Here we propose experiments designed to identify factors that drive this conformational change in solution and to link those factors with the molecular mechanism of TRC40 function.