EFFECTS OF PREPROTEIN & NUCLEOTIDE BINDING ON SOLUTION STRUCTURE OF SECA ATPASE

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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. The SecA ATPase is the engine of preprotein translocase, an essential protein translocation system in prokaryotes. SecA interacts with the protein-conducting pore, SecYEG, and uses the energy of ATP to translocate unfolded preproteins across the cytoplasmic membrane. To push the preprotein through SecYEG, SecA undergoes repeated cycles of preprotein binding and release, which are linked to ATP binding and hydrolysis. The goal of the SAXS experiments is to investigate the conformational changes that take place in SecA when preprotein and/or nucleotides are bound. In addition to being an engine for translocation, SecA is a chaperone in the sense that it interacts only with unfolded preprotein, and this has made detailed biophysical studies of the SecA-preprotein interaction impossible because the preprotein will either refold or aggregate during the experiment. To overcome this technical hurdle, we have used immobilized peptide libraries to find short peptide sequences that bind to SecA with high affinity. Using this technology, we have identified a number of 15 residue peptides that are highly soluble and bind to SecA in a nucleotide-dependent manner. We believe that there are significant conformational changes in SecA when it binds these peptides. SecA is a multi-domain, modular protein (Fig. 1 in attached SecA-SAXS.pdf). Based on a variety of structural (Fig. 2) and biochemical data (not shown), we believe the preprotein is bound between the nucleotide binding domains and the preprotein binding domain, requiring conformational changes in SecA (Fig. 3); an example of the attendant changes in SAXS is provided in Fig. 4. Taking advantage of the modular nature of SecA, we have 4 different constructs to use for SAXS experiments: the effect of preprotein and nucleotide binding on the structure of each construct will allow us to map the inter-domain movements that take place and show how ATP binding and hydrolysis are coupled to preprotein binding.