SOLUTION STRUCTURE OF THE SIGNAL RECOGNITION PARTICLE FROM T AQUATICUS: A SAXS

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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 signal recognition particle (SRP) and its membrane-associated receptor (SR) constitute an evolutionary conserved macromolecular ribonucleoproteic complex that catalyzes targeting of nascent secretory and membrane proteins to the protein translocation apparatus. The SRP is in charge of directing ribosomes which are currently translating proteins destined for either secretion or membrane integration to the endoplasmic reticulum or plasma membrane. The receptor is responsible for the targeting of the ribosome-nascent protein-SRP complex to the membrane translocation machinery. The components of the SRP pathway and the essential steps of the molecular mechanism of SRP-dependent protein targeting are conserved in all three kingdoms of life. The thermostable signal recognition particle from the eubacteria Thermus aquaticus (Taq) is a ribonucleic acid-protein complex composed of the SRP-RNA, which is 113 nucleotides long (35 kDa), and two proteins, the receptor subunit FtsY (33 kDal) and the RNA and signal sequence-binding subunit Ffh (48 kDa). In addition both proteins are structurally related GTPases and GTP-dependent GTPase-activating proteins (GAPs). The ribosome-nascent protein-SRP complex interaction with its receptor is also GTP dependent. GTP hydrolysis by the SRP-SR complex dissociates this complex, allowing a new round of targeting. All components of the Taq-SRP are overexpressed and soluble and have been extensively characterized by gel filtration, dynamic light scattering and analytical ultracentrifugation, in terms of homogeneity and stability. The aim of this biophysical study by SAXS is to charactize the shapes and the conformations of the Taq-SRP and its subcomponents in solution. Since the crystal structures of the isolated subunits are available, using SAXS techniques we wish to construct low-resolution models of the binary (FtsY/Ffh) and ternary (FtsY/Ffh/SRP-RNA) complexes whose structures are still unknown.