Crystallization and preliminary X-ray diffraction studies on the conservedGTPase domain of the signal recognition particle from Acidianus ambivalens

The signal recognition particle (SRP) of bacteria consists of only one prot ein, known as Ffh or the SRP54 homologue, which forms a complex with 4.5S R NA. It also binds to signal peptides and contains a GTPase which displays i nteresting differences to Ras GTPases. The conserved NG-domain of Ffh from the archaebacterium Acidianus ambivalens was cloned and overexpressed with a C-terminal His tag in Escherichia coil. Crystallization experiments of th e native protein as well as of the Thr112Ala mutant, which is deficient in GTP hydrolysis, resulted in crystals suitable for X-ray diffraction. The cr ystals belong to the orthorhombic space group C222(1), with unit-cell param eters a = 64.5, b = 128.3, c = 72.0 Angstrom. At cryogenic temperatures, th e crystals diffracted to a resolution limit of 2.8 Angstrom using a rotatin g-anode generator and contain one molecule per asymmetric unit. A native da ta set has been collected using synchrotron radiation to around 2.0 Angstro m resolution. Selenomethionine protein was produced; its crystals diffract in-house to about 2.8 Angstrom resolution.