Cotranslational targeting and insertion of proteins into the protein conducting channel (PCC) of the endoplasmic reticulum (ER) requires the action of three interacting GTPases; SRP54, and the SRP receptor heterodimer SRalpha, and SRbeta. The SRbeta is one component of this pathway for which little is known. The proposed research will use a combination of biochemical analysis and X-ray crystallography to characterize the function and the structure of SRbeta alone and as a unit of the SRP receptor. The identification of regions involved in the interactions and transmission of signals will serve as a major tool in elucidating the mechanisms by which the combination of GTPases regulates protein translocation.