Satellite panicum mosaic virus (SPMV) has been purified from pearl millet and obtained in a variety of different crystal forms, at least two of which appear suitable for high resolution X-ray diffraction analysis. The first is of cubic space group P4(2)32 with a = b = c = 183.1 A and two virus particles in the unit cell. The second is of a primitive orthorhombic space group with a = 166.1 A, b = 266.7 A and c = 269.1 A, with four virus particles in the unit cell. While the cubic crystal has as its asymmetric unit one twelfth of the icosahedron, or five capsid protein subunits, the asymmetric unit of the orthorhombic crystals is an entire particle. The cubic crystals diffract to at least 2.8 A resolution. We have also succeeded in crystallizing, but not yet characterizing the master virus, PMV.

The Fab fragment of an antibody, made against an E2-specific feline infectious peritonitis virus neutralizing antibody, has been crystallized in a form suitable for X-ray diffraction analysis from PEG 4000 using vapor diffusion methods. The Fab fragment crystals diffract to about 2.9 A resolution and are of triclinic space group P1. Unit cell dimensions, by which the reciprocal lattice can be indexed, are a = 57.16 A, b = 70.85 A, c = 75.81 A, alpha = 85.11 degrees, beta = 121.28 degrees and gamma = 116.33 degrees. There are two Fab fragments comprising the asymmetric unit of the crystals. The presence of a pseudo-mirror plane in the diffraction pattern suggests the presence of at least an approximate dyad axis relating the two Fab fragments within the asymmetric unit.

Cotranslational targeting of membrane and secretory proteins is mediated by the universally conserved signal recognition particle (SRP). Together with its receptor (SR), SRP mediates the guanine triphosphate (GTP)-dependent delivery of translating ribosomes bearing signal sequences to translocons on the target membrane. Here, we present the crystal structure of the SRP:SR complex at 3.9 angstrom resolution and biochemical data revealing that the activated SRP:SR guanine triphosphatase (GTPase) complex binds the distal end of the SRP hairpin RNA where GTP hydrolysis is stimulated. Combined with previous findings, these results suggest that the SRP:SR GTPase complex initially assembles at the tetraloop end of the SRP RNA and then relocalizes to the opposite end of the RNA. This rearrangement provides a mechanism for coupling GTP hydrolysis to the handover of cargo to the translocon.

The conserved signal recognition particle (SRP) cotranslationally delivers ~30% of the proteome to the eukaryotic endoplasmic reticulum (ER). The molecular mechanism by which eukaryotic SRP transitions from cargo recognition in the cytosol to protein translocation at the ER is not understood. Here, structural, biochemical, and single-molecule studies show that this transition requires multiple sequential conformational rearrangements in the targeting complex initiated by guanosine triphosphatase (GTPase)-driven compaction of the SRP receptor (SR). Disruption of these rearrangements, particularly in mutant SRP54_G226E linked to severe congenital neutropenia, uncouples the SRP/SR GTPase cycle from protein translocation. Structures of targeting intermediates reveal the molecular basis of early SRP-SR recognition and emphasize the role of eukaryote-specific elements in regulating targeting. Our results provide a molecular model for the structural and functional transitions of SRP throughout the targeting cycle and show that these transitions provide important points for biological regulation that can be perturbed in genetic diseases.