Biomedical subjects

R Holland Cheng

Publications and source records attributed to R Holland Cheng.

23 records · Page 2Linked to original sources

PubMed2003

The atomic structure of rice dwarf virus reveals the self-assembly mechanism of component proteins.

Rice dwarf virus (RDV), the causal agent of rice dwarf disease, is a member of the genus Phytoreovirus in the family Reoviridae. RDV is a double-shelled virus with a molecular mass of approximately 70 million Dalton. This virus is widely prevalent and is one of the viruses that cause the most economic damage in many Asian countries. The atomic structure of RDV was determined at 3.5 A resolution by X-ray crystallography. The double-shelled structure consists of two different proteins, the core protein P3 and the outer shell protein P8. The atomic structure shows structural and electrostatic complementarities between both homologous (P3-P3 and P8-P8) and heterologous (P3-P8) interactions, as well as overall conformational changes found in P3-P3 dimer caused by the insertion of amino-terminal loop regions of one of the P3 protein into the other. These interactions suggest how the 900 protein components are built into a higher-ordered virus core structure.

Bluetongue virus↗

PubMed2003

Structural analysis of human rhinovirus complexed with ICAM-1 reveals the dynamics of receptor-mediated virus uncoating.

Intercellular adhesion molecule 1 (ICAM-1) functions as the cellular receptor for the major group of human rhinoviruses, being not only the target of viral attachment but also the mediator of viral uncoating. The configurations of HRV3-ICAM-1 complexes prepared both at 4 degrees C and physiological temperature (37 degrees C) were analyzed by cryoelectron microscopy and image reconstruction. The particle diameters of two complexes (with and without RNA) representing uncoating intermediates generated at 37 degrees C were each 4% larger than that of those prepared at 4 degrees C. The larger virus particle arose by an expansive movement of the capsid pentamers along the fivefold axis, which loosens interprotomer contacts, particularly at the canyon region where the ICAM-1 receptor bound. Particle expansion required receptor binding and preceded the egress of the viral RNA. These observations suggest that receptor-mediated uncoating could be a consequence of restrained capsid motion, where the bound receptors maintain the viral capsid in an expanded open state for subsequent genome release.

Binding Sites↗

PubMed2002

Prefusion rearrangements resulting in fusion Peptide exposure in Semliki forest virus.

Semliki Forest virus (SFV), like many enveloped viruses, takes advantage of the low pH in the endosome to convert into a fusion-competent configuration and complete infection by fusion with the endosomal membrane. Unlike influenza virus, carrying an N-terminal fusion peptide, SFV represents a less-well understood fusion principle involving an endosequence fusion peptide. To explore the series of events leading to a fusogenic configuration of the SFV, we exposed the virus to successive acidification, mimicking endosomal conditions, and followed structural rearrangements at probed sensor surfaces. Thus revealed, the initial phase involves a transient appearance of a non-linear neutralizing antibody epitope in the fusion protein, E1. Concurrent with the disappearance of this epitope, a set of masked sequences in proteins E1 and E2 became exposed. When pH reached 6.0-5.9 the virion transformed into a configuration of enlarged diameter with the fusion peptide optimally exposed. Simultaneously, a partly hidden sequence close to the receptor binding site in E2 became fully uncovered. At this presumably fusogenic stage, maximally 80 fusion peptide-identifying antibody Fab fragments could be bound per virion, i.e. one ligand per three copies of the fusion protein. The phenomena observed are discussed in terms of alphavirus structure and reported functional domains.

Amino Acid Sequence↗

PubMed2002

Acid-induced movements in the glycoprotein shell of an alphavirus turn the spikes into membrane fusion mode.

In the icosahedral (T = 4) Semliki Forest virus, the envelope protomers, i.e. E1-E2 heterodimers, make one-to-one interactions with capsid proteins below the viral lipid bilayer, transverse the membrane and form an external glycoprotein shell with projections. The shell is organized by protomer domains interacting as hexamers and pentamers around shell openings at icosahedral 2- and 5-fold axes, respectively, and the projections by other domains associating as trimers at 3- and quasi 3-fold axes. We show here, using cryo- electron microscopy, that low pH, as occurs in the endosomes during virus uptake, results in the relaxation of protomer interactions around the 2- and the 5-fold axes in the shell, and movement of protomers towards 3- and quasi 3-fold axes in a way that reciprocally relocates their putative E1 and E2 domains. This seemed to be facilitated by a trimerization of transmembrane segments at the same axes. The alterations observed help to explain several key features of the spike-mediated membrane fusion reaction, including shell dissolution, heterodimer dissociation, fusion peptide exposure and E1 homotrimerization.

Alkanesulfonic Acids↗

PubMed1995

DNA packaging intermediates of bacteriophage φX174

BACKGROUND: Like many viruses, bacteriophage phi X174 packages its DNA genome into a procapsid that is assembled from structural intermediates and scaffolding proteins. The procapsid contains the structural proteins F, G and H, as well as the scaffolding proteins B and D. Provirions are formed by packaging of DNA together with the small internal J proteins, while losing at least some of the B scaffolding proteins. Eventually, loss of the D scaffolding proteins and the remaining B proteins leads to the formation of mature virions. RESULTS: phi X174 108S 'procapsids' have been purified in milligram quantities by removing 114S (mature virion) and 70S (abortive capsid) particles from crude lysates by differential precipitation with polyethylene glycol. 132S 'provirions' were purified on sucrose gradients in the presence of EDTA. Cryo-electron microscopy (cryo-EM) was used to obtain reconstructions of procapsids and provirions. Although these are very similar to each other, their structures differ greatly from that of the virion. The F and G proteins, whose atomic structures in virions were previously determined from X-ray crystallography, were fitted into the cryo-EM reconstructions. This showed that the pentamer of G proteins on each five-fold vertex changes its conformation only slightly during DNA packaging and maturation, whereas major tertiary and quaternary structural changes occur in the F protein. The procapsids and provirions were found to contain 120 copies of the D protein arranged as tetramers on the two-fold axes. DNA might enter procapsids through one of the 30 A diameter holes on the icosahedral three-fold axes. CONCLUSIONS: Combining cryo-EM image reconstruction and X-ray crystallography has revealed the major conformational changes that can occur in viral assembly. The function of the scaffolding proteins may be, in part, to support weak interactions between the structural proteins in the procapsids and to cover surfaces that are subsequently required for subunit-subunit interaction in the virion. The structures presented here are, therefore, analogous to chaperone proteins complexed with folding intermediates of a substrate.

Bacteriophage phi X 174↗