Viral capsid assembly and stability in tailed, dsDNA phage and Herpesviridae are achieved by various means including chemical cross-links (unique to HK97), or auxiliary proteins (lambda, T4, ϕ29, and herpesviruses). All these viruses have coat proteins (CP) with a conserved, HK97-like core structure. We used a combination of trypsin digestion, gold-labeling, cryo-electron microscopy, 3D image reconstruction, and comparative modeling to derive two independent, pseudo-atomic models of bacteriophage P22 CP: before and after maturation. P22 capsid stabilization results from inter-subunit interactions among N-terminal helices and an extensive “P-loop”, which obviate the need for cross-links or auxiliary proteins. P22 CP also has a telokin-like, Ig domain that stabilizes the monomer fold so that assembly may proceed via individual subunit addition rather than via preformed capsomers as occurs in HK97. Hence, the P22 CP structure may be a paradigm for understanding how monomers assemble in viruses like ϕ29 and HSV-1.

Summary For many (if not all) bacterial and archaeal tailed viruses and eukaryotic Herpesvirdae the HK97-fold serves as the major architectural element in icosahedral capsid formation while still enabling the conformational flexibility required during assembly and maturation. Auxiliary proteins or Δ-domains strictly control assembly of multiple, identical, HK97-like subunits into procapsids with specific icosahedral symmetries, rather than aberrant non-icosahedral structures. Procapsids are precursor structures that mature into capsids in a process involving release of auxiliary proteins (or cleavage of Δ-domains), dsDNA packaging, and conformational rearrangement of the HK97-like subunits. Some coat proteins built on the ubiquitous HK97-fold also have accessory domains or loops that impart specific functions, such as increased monomer, procapsid, or capsid stability. In this review, we analyze the numerous HK97-like coat protein structures that are emerging in the literature (over 40 at time of writing) by comparing their topology, additional domains, and their assembly and misassembly reactions.

SUMMARY Some capsid proteins built on the ubiquitous HK97-fold have accessory domains that impart specific functions. Bacteriophage P22 coat protein has a unique inserted I-domain. Two prior I-domain models from sub-nanometer cryoEM reconstructions differed substantially. Therefore, the NMR structure of the I-domain was determined, which also was used to improve cryoEM models of coat protein. The I-domain has an anti-parallel 6-stranded β-barrel fold, previously not observed in HK97-fold accessory domains. The D-loop, which is dynamic both in the isolated I-domain and intact monomeric coat protein, forms stabilizing salt bridges between adjacent capsomers in procapsids. A newly described S-loop is important for capsid size determination, likely through intra-subunit interactions. Ten of eighteen coat protein temperature-sensitive-folding substitutions are in the I-domain, indicating its importance in folding and stability. Several are found on a positively charged face of the β-barrel that anchors the I-domain to a negatively charged surface of the coat protein HK97-core.

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The bacterial Mfd ATPase is increasingly recognized as a general transcription factor that participates in the resolution of transcription conflicts with other processes/roadblocks. This function stems from Mfd's ability to preferentially act on stalled RNA polymerases (RNAPs). However, the mechanism underlying this preference and the subsequent coordination between Mfd and RNAP have remained elusive. Here, using a novel real-time translocase assay, we unexpectedly discovered that Mfd translocates autonomously on DNA. The speed and processivity of Mfd dictate a "release and catch-up" mechanism to efficiently patrol DNA for frequently stalled RNAPs. Furthermore, we showed that Mfd prevents RNAP backtracking or rescues a severely backtracked RNAP, allowing RNAP to overcome stronger obstacles. However, if an obstacle's resistance is excessive, Mfd dissociates the RNAP, clearing the DNA for other processes. These findings demonstrate a remarkably delicate coordination between Mfd and RNAP, allowing efficient targeting and recycling of Mfd and expedient conflict resolution.

SummaryWe have investigated determinants of polyhead formation in bacteriophage P22 in order to understand the molecular mechanism by which coat protein assembly goes astray. Polyhead assembly is caused by amino acid substitutions in coat protein at position 170, which is located in the b-hinge. In vivo scaffolding protein does not correct polyhead assembly by F170A or F170K coat proteins, but does for F170L. All F170 variants bind scaffolding protein more weakly than wild-type as observed by affinity chromatography with scaffolding protein-agarose and scaffolding protein shell re-entry experiments. Electron cryomicroscopy and three-dimensional image reconstructions of F170A and F170K empty procapsid shells showed that there is a decreased flexibility of the coat subunits relative to wild-type. This was confirmed by limited proteolysis and protein sequencing, which showed increased protection of the A-domain. Our data support the conclusion that the decrease in flexibility of the A-domain leads to crowding of the subunits at the centre of the pentons, thereby favouring the hexon configuration during assembly. Thus, correct coat protein interactions with scaffolding protein and maintenance of sufficient coat protein flexibility are crucial for proper P22 assembly. The coat protein b-hinge region is the major determinant for both features.

The stationary phase promoter specificity subunit σ S (RpoS) is delivered to the ClpXP machinery for degradation dependent on the adaptor RssB. This adaptor-specific degradation of σ S provides a major point for regulation and transcriptional reprogramming during the general stress response. RssB is an atypical response regulator and the only known ClpXP adaptor that is inhibited by multiple but dissimilar antiadaptors (IraD, IraP, and IraM). These are induced by distinct stress signals and bind to RssB in poorly understood manners to achieve stress-specific inhibition of σ S turnover. Here we present the first crystal structure of RssB bound to an antiadaptor, the DNA damage-inducible IraD. The structure reveals that RssB adopts a compact closed architecture with extensive interactions between its N-terminal and C-terminal domains. The structural data, together with mechanistic studies, suggest that RssB plasticity, conferred by an interdomain glutamate-rich flexible linker, is critical for regulation of σ S degradation. Structural modulation of interdomain linkers may thus constitute a general strategy for tuning response regulators.

SummaryEighteen single amino acid substitutions in phage P22 coat protein cause temperature-sensitive folding defects (tsf). Three intragenic global suppressor (su) substitutions (D163G, T166I and F170L), localized to a flexible loop, rescue the folding of several tsf coat proteins. Here we investigate the su substitutions in the absence of the original tsf substitutions. None of the su variant coat proteins displayed protein folding defects. Individual su substitutions had little effect on phage production in vivo; yet double and triple combinations resulted in a cold-sensitive (cs) phenotype, consistent with a defect in assembly. During virus assembly and maturation, conformational switching of capsid subunits is required when chemically identical capsid subunits form an icosahedron. Analysis of double-and triple-su phage-infected cell lysates by negative-stain electron microscopy reveals an increase in aberrant structures at the cs temperature. In vitro assembly of F170L coat protein causes production of polyheads, never seen before in phage P22. Purified procapsids composed of all of the su coat proteins showed defects in expansion, which mimics maturation in vitro. Our results suggest that a previously identified surface-exposed loop in coat protein is critical in conformational switching of subunits during both procapsid assembly and maturation.