PubMed Health⌕ Search

Biomedical subjects

R Golmohammadi

Publications and source records attributed to R Golmohammadi.

4 recordsLinked to original sources
PubMed1996

The crystal structure of bacteriophage Q beta at 3.5 A resolution.

BACKGROUND: The capsid protein subunits of small RNA bacteriophages form a T = 3 particle upon assembly and RNA encapsidation. Dimers of the capsid protein repress translation of the replicase gene product by binding to the ribosome binding site and this interaction is believed to initiate RNA encapsidation. We have determined the crystal structure of phage Q beta with the aim of clarifying which factors are the most important for particle assembly and RNA interaction in the small phages. RESULTS: The crystal structure of bacteriophage Q beta determined at 3.5 A resolution shows that the capsid is stabilized by disulfide bonds on each side of the flexible loops that are situated around the fivefold and quasi-sixfold axes. As in other small RNA phages, the protein capsid is constructed from subunits which associate into dimers. A contiguous ten-stranded antiparallel beta sheet facing the RNA is formed in the dimer. The disulfide bonds lock the constituent dimers of the capsid covalently in the T = 3 lattice. CONCLUSIONS: The unusual stability of the Q beta particle is due to the tight dimer interactions and the disulfide bonds linking each dimer covalently to the rest of the capsid. A comparison with the structure of the related phage MS2 shows that although the fold of the Q beta coat protein is very similar, the details of the protein-protein interactions are completely different. The most conserved region of the protein is at the surface, which, in MS2, is involved in RNA binding.

Allolevivirus↗
PubMed1996

Crystal structures of MS2 capsids with mutations in the subunit FG loop.

The loop between the F and G beta strands (FG loop) of the bacteriophage MS2 coat protein subunit forms inter-subunit contacts around the 5-fold and 3-fold (quasi 6-fold) axes of the T=3 protein shell. In capsids, the loop is found in two very different conformations, one in B subunits, which form the 5-fold contact, and one in A and C subunits, which form the quasi 6-fold contact. One proline residue, Pro78, is strictly conserved in the coat protein of all related bacteriophages, and in the case of MS2 this proline residue is preceded by a cis peptide bond in the B subunit. In order to probe the role of the FG loop in capsid assembly, we have determined the crystal structures of two MS2 capsids, formed by coat proteins with mutations at two positions in the FG loop, P78N or E76D. These mutants show conformational changes in the FG loops that explain the reduced temperature stability of the capsids. The P78N mutant has a normal trans peptide bond at position 78.

Capsid↗
PubMed1993

The refined structure of bacteriophage MS2 at 2.8 A resolution.

Bacteriophage MS2 is an icosahedral virus with 180 copies of a coat protein forming a shell around a single-stranded RNA molecule. The coat protein subunits form a lattice with the triangulation number T = 3. The coat protein has a fold which is different from the fold of all other viral coat proteins so far known. It consists of a five-stranded beta sheet facing the inside of the particle, and a hairpin and two helices on the outside. The crystal structure has been refined at 2.8 A resolution. The final R-factor was 0.189 for reflections with F > 2 sigma, and the root-mean-square deviation from idealized bond lengths and bond angles was 0.015 A and 2.9 degrees, respectively. The three chemically identical conformers A, B and C are largely similar. The B conformer has a unique conformation in one loop, which is involved in 5-fold interactions, while the A and C conformers, which are involved in the quasi-6-fold contacts, are similar throughout the structure. One cis-proline has been identified in the B conformer but the corresponding prolines in A and C are of the trans isomer. This residue is conserved within small RNA coliphages and it is proposed that this isomerization enables a less elongated loop (FG) around the 5-fold axis, thus creating a channel. The extensive dimer contact supports the idea of dimers as initial building blocks. An assembly pathway is proposed where five dimers converge into a pentamer and 12 pentamers are linked together with free dimers creating a complete particle.

Amino Acid Sequence↗