Structural and molecular basis of mismatch correction and ribavirin excision from coronavirus RNA.

Coronaviruses (CoVs) stand out among RNA viruses because of their unusually large genomes (∼30 kb) associated with low mutation rates. CoVs code for nsp14, a bifunctional enzyme carrying RNA cap guanine N7-methyltransferase (MTase) and 3'-5' exoribonuclease (ExoN) activities. ExoN excises nucleotide mismatches at the RNA 3'-end in vitro, and its inactivation in vivo jeopardizes viral genetic stability. Here, we demonstrate for severe acute respiratory syndrome (SARS)-CoV an RNA synthesis and proofreading pathway through association of nsp14 with the low-fidelity nsp12 viral RNA polymerase. Through this pathway, the antiviral compound ribavirin 5'-monophosphate is significantly incorporated but also readily excised from RNA, which may explain its limited efficacy in vivo. The crystal structure at 3.38 Šresolution of SARS-CoV nsp14 in complex with its cofactor nsp10 adds to the uniqueness of CoVs among RNA viruses: The MTase domain presents a new fold that differs sharply from the canonical Rossmann fold.

Structural characterization of the N-terminal part of the MERS-CoV nucleocapsid by X-ray diffraction and small-angle X-ray scattering.

The N protein of coronaviruses is a multifunctional protein that is organized into several domains. The N-terminal part is composed of an intrinsically disordered region (IDR) followed by a structured domain called the N-terminal domain (NTD). In this study, the structure determination of the N-terminal region of the MERS-CoV N protein via X-ray diffraction measurements is reported at a resolution of 2.4 Å. Since the first 30 amino acids were not resolved by X-ray diffraction, the structural study was completed by a SAXS experiment to propose a structural model including the IDR. This model presents the N-terminal region of the MERS-CoV as a monomer that displays structural features in common with other coronavirus NTDs.

The SH3 regulatory domain of the hematopoietic cell kinase Hck binds ELMO via its polyproline motif.

Eukaryotic EnguLfment and cell MOtility (ELMO) proteins form an evolutionary conserved family of regulators involved in small GTPase dependent actin remodeling processes that regulates the guanine exchange factor activity of some of the Downstream Of CrK (DOCK) family members. Gathered data strongly suggest that DOCK activation by ELMO and the subsequent signaling result from a subtle balance in the binding of partners to ELMO. Among its putative upward modulators, the Hematopoietic cell kinase (Hck), a member of the Src kinase superfamily, has been identified as a binding partner and a specific tyrosine kinase for ELMO1. Indeed, Hck is implicated in distinct molecular signaling pathways governing phagocytosis, cell adhesion, and migration of hematopoietic cells. Although ELMO1 has been shown to interact with the regulatory Src Homology 3 (SH3) domain of Hck, no direct evidence indicating the mode of interaction between Hck and ELMO1 have been provided in the literature. In the present study, we report convergent pieces of evidence that demonstrate the specific interaction between the SH3 domain of Hck and the polyproline motif of ELMO1. Our results also suggest that the tyrosine-phosphorylation state of ELMO1 tail might act as a putative modulator of Hck kinase activity towards ELMO1 that in turn participates in DOCK180 activation and further triggers subsequent signaling towards actin remodeling.

Insights into RNA synthesis, capping, and proofreading mechanisms of SARS-coronavirus.

The successive emergence of highly pathogenic coronaviruses (CoVs) such as the Severe Acute Respiratory Syndrome (SARS-CoV) in 2003 and the Middle East Respiratory Syndrome Coronavirus (MERS-CoV) in 2012 has stimulated a number of studies on the molecular biology. This research has provided significant new insight into functions and activities of the replication/transcription multi-protein complex. The latter directs both continuous and discontinuous RNA synthesis to replicate and transcribe the large coronavirus genome made of a single-stranded, positive-sense RNA of ∼30 kb. In this review, we summarize our current understanding of SARS-CoV enzymes involved in RNA biochemistry, such as the in vitro characterization of a highly active and processive RNA polymerase complex which can associate with methyltransferase and 3'-5' exoribonuclease activities involved in RNA capping, and RNA proofreading, respectively. The recent discoveries reveal fascinating RNA-synthesizing machinery, highlighting the unique position of coronaviruses in the RNA virus world.

Coiled-coil deformations in crystal structures: the measles virus phosphoprotein multimerization domain as an illustrative example.

The structures of two constructs of the measles virus (MeV) phosphoprotein (P) multimerization domain (PMD) are reported and are compared with a third structure published recently by another group [Communie et al. (2013), J. Virol. 87, 7166-7169]. Although the three structures all have a tetrameric and parallel coiled-coil arrangement, structural comparison unveiled considerable differences in the quaternary structure and unveiled that the three structures suffer from significant structural deformation induced by intermolecular interactions within the crystal. These results show that crystal packing can bias conclusions about function and mechanism based on analysis of a single crystal structure, and they challenge to some extent the assumption according to which coiled-coil structures can be reliably predicted from the amino-acid sequence. Structural comparison also highlighted significant differences in the extent of disorder in the C-terminal region of each monomer. The differential flexibility of the C-terminal region is also supported by size-exclusion chromatography and small-angle X-ray scattering studies, which showed that MeV PMD exists in solution as a dynamic equilibrium between two tetramers of different compaction. Finally, the possible functional implications of the flexibility of the C-terminal region of PMD are discussed.

The C-terminal polyproline-containing region of ELMO contributes to an increase in the life-time of the ELMO-DOCK complex.

The eukaryotic Engulfment and CellMotility (ELMO) proteins form an evolutionary conserved family of key regulators which play a central role in Rho-dependent biological processes such as engulfment and cell motility/migration. ELMO proteins interact with a subset of Downstream of Crk (DOCK) family members, a new type of guanine exchange factors (GEF) for Rac and cdc42 GTPases. The physiological function of DOCK is to facilitate actin remodeling, a process which occurs only in presence of ELMO. Several studies have determined that the last 200 C-terminal residues of ELMO1 and the first 180 N-terminal residues of DOCK180 are responsible for the ELMO-DOCK interaction. However, the precise role of the different domains and motifs identified in these regions has remained elusive. Divergent functional, biochemical and structural data have been reported regarding the contribution of the C-terminal end of ELMO, comprising its polyproline motif, and of the DOCK SH3 domain. In the present study, we have investigated the contribution of the C-terminal end of ELMO1 to the interaction between ELMO1 and the SH3 domain of DOCK180 using nuclear magnetic resonance spectroscopy and surface plasmon resonance. Our data presented here demonstrate the ability of the SH3 domain of DOCK180 to interact with ELMO1, regardless of the presence of the polyproline-containing C-terminal end. However, the presence of the polyproline region leads to a significant increase in the half-life of the ELMO1-DOCK180 complex, along with a moderate increase on the affinity.