The signal recognition particle (SRP) is an evolutionarily conserved ribonucleoprotein that targets 'nascent secreted and membrane proteins to the endoplasmic reticulum. The SRP plays a role in the secretion of numerous clinically relevant proteins, and an understanding of its structural and functional mechanisms is of fundamental importance for understanding the biology of the co-translational secretion pathway in the the eukaryotic cell. One protein of the SRP, termed SRP54, integrates two key activities of the particle - the recognition of the N-terminal signal peptide, and the GTP-dependent formation of a targeting complex with the SRP receptor (SR) at the membrane. Crystal structures of the signal recognition and GTPase domains of the prokaryotic homolog of SRP54 allow the construction of a three-dimensional model of the eukaryotic protein. On this basis a pair of cysteine residues that are universally conserved in the eukaryotic SRP54 can be mapped to opposite sides of the GTPase active site pocket. The functional role of these two residues is completely unknown. We hypothesize that the two cysteines, perhaps regulated by other factors such as the cellular redox state, modulate or alter the GTPase cycle of the SRP. As a first step in this work, therefore, we aim to characterize the behavior of the GTPase domain of SRP54 with respect to redox and disulfide states. Subsequent work is directed towards understanding the role of the cysteines in vivo, with a long term goal of understanding the structural basis for their function and determining their biological significance. Although the project is somewhat high risk, it has the potential to reveal an unanticipated and significant new regulatory mechanism of the SRP.
