CRYSTAL STRUCTURES OF SIGNAL RECOGNITION PARTICLE (SRP) SRP9 PROTEIN AND SRP[[Phi]]14/9 FUSION PROTEIN. Darcy E.A. Birse¹, Anders Åberg¹, Kieron Brown¹, Sylvie Doublié¹*, Ulrike Kapp¹, Katharina Strub² and Stephen Cusack¹. ¹European Molecular Biology Laboratory (EMBL-Grenoble)-Grenoble Outstation, c/o ILL, 156X, 38042 Grenoble Cedéx 9, France, ² Département de Biologie Cellulaire, Université de Genève, Sciences III, CH-1211 Geneva 4, Switzerland, *Department of Biological Chemistry and Molecular Pharmacology, Harvard University Medical School, Boston, Mass., 02115, U.S.A

The mammalian Signal Recognition Particle (SRP) is a cytoplasmic ribonucleoprotein particle (RNP) that plays an essential role in the targeting of secretory and membrane proteins to the rough endoplasmic reticulum (RER). Targeting occurs co-translationally and translocation across the RER membrane begins before polypeptide synthesis is complete.

The mammalian SRP is an 11S cytoplasmic RNP which consists of six polypeptides (SRP9, SRP14, SRP19, SRP54, SRP68, SRP72) and a single RNA molecule.

SRP9 protein has been crystallized in the point group 321/622 with cell parameters a=b=64.0 Å, c=110.5 Å. Using synchrotron radiation on single flash-frozen crystals, a complete data set showing diffraction beyond 2.3 Å resolution with a R_sym on intensities of 4.5% was collected. MAD data at four wavelengths was collected on selenomethionated protein at the MAD beamline (BL19 ESRF) complete to 2.8 Å resolution with R_sym on intensities of 6.0%.

For structural and functional studies a fusion protein, denoted SRP[[Phi]]14/9, has been constructed which can functionally replace the SRP9/14 heterodimeric subunit in the SRP. The SRP[[Phi]]14/9 has been crystallized in the space group P4₁22/P4₃22 with cell parameters a=b=69.7 Å, c=95.7 Å. A synchrotron data set (BL19 ESRF) on a single flash-frozen crystal, complete to 2.8 Å resolution with R_sym on intensities of 4.4% has been collected. Selenomethionyl SRP[[Phi]]14/9 has also been crystallized for use in phase determination by MIR and/or MAD methods.