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Membrane topology of murine coronavirus replicase nonstructural protein 3.

ABSTRACT: Mouse hepatitis virus (MHV) is a member of the family Coronaviridae. These positive strand RNA viruses encode a replicase polyprotein that is processed into 16 nonstructural proteins (nsps). The nsps assemble with membranes to generate double membrane vesicles, which are the sites of viral RNA synthesis. MHV nsp3 contains multiple domains including two papain-like protease domains, PLP1 and PLP2, and a predicted transmembrane (TM) domain. In this study, we determined the membrane topology of nsp3-TM and showed that TM-mediated tethering of PLP2 is important for processing at cleavage site 3. Biochemical analysis revealed that nsp3 is an integral membrane protein that is inserted into the endoplasmic reticulum (ER) membranes co-translationally and glycosylated at asparagine-2357. Proteinase K digestion experiments indicate that the TM domain of nsp3 has 4 membrane-spanning helices. We show that nsp3-TM is sufficient in mediating ER membrane association of a cytosolic protein. This study is the first detailed analysis of the topology and function of the coronavirus nsp3 TM domain.

SUBMITTER: Kanjanahaluethai A

PROVIDER: S-EPMC1925034 | biostudies-literature | 2007 May

REPOSITORIES: biostudies-literature

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Membrane topology of murine coronavirus replicase nonstructural protein 3.

Kanjanahaluethai Amornrat A Chen Zhongbin Z Jukneliene Dalia D Baker Susan C SC

Virology 20070112 2

Mouse hepatitis virus (MHV) is a member of the family Coronaviridae. These positive strand RNA viruses encode a replicase polyprotein that is processed into 16 nonstructural proteins (nsps). The nsps assemble with membranes to generate double membrane vesicles, which are the sites of viral RNA synthesis. MHV nsp3 contains multiple domains including two papain-like protease domains, PLP1 and PLP2, and a predicted transmembrane (TM) domain. In this study, we determined the membrane topology of nsp ...[more]

PMID: 17222884

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Project description:UnlabelledCoronaviruses, the largest RNA viruses, have a complex program of RNA synthesis that entails genome replication and transcription of subgenomic mRNAs. RNA synthesis by the prototype coronavirus mouse hepatitis virus (MHV) is carried out by a replicase-transcriptase composed of 16 nonstructural protein (nsp) subunits. Among these, nsp3 is the largest and the first to be inserted into the endoplasmic reticulum. nsp3 comprises multiple structural domains, including two papain-like proteases (PLPs) and a highly conserved ADP-ribose-1″-phosphatase (ADRP) macrodomain. We have previously shown that the ubiquitin-like domain at the amino terminus of nsp3 is essential and participates in a critical interaction with the viral nucleocapsid protein early in infection. In the current study, we exploited atypical expression schemes to uncouple PLP1 from the processing of nsp1 and nsp2 in order to investigate the requirements of nsp3 domains for viral RNA synthesis. In the first strategy, a mutant was created in which replicase polyprotein translation initiated with nsp3, thereby establishing that complete elimination of nsp1 and nsp2 does not abolish MHV viability. In the second strategy, a picornavirus autoprocessing element was used to separate a truncated nsp1 from nsp3. This provided a platform for further dissection of amino-terminal domains of nsp3. From this, we found that catalytic mutation of PLP1 or complete deletion of PLP1 and the adjacent ADRP domain was tolerated by the virus. These results showed that neither the PLP1 domain nor the ADRP domain of nsp3 provides integral activities essential for coronavirus genomic or subgenomic RNA synthesis.ImportanceThe largest component of the coronavirus replicase-transcriptase complex, nsp3, contains multiple modules, many of which do not have clearly defined functions in genome replication or transcription. These domains may play direct roles in RNA synthesis, or they may have evolved for other purposes, such as to combat host innate immunity. We initiated a dissection of MHV nsp3 aimed at identifying those activities or structures in this huge molecule that are essential to replicase activity. We found that both PLP1 and ADRP could be entirely deleted, provided that the requirement for proteolytic processing by PLP1 was offset by an alternative mechanism. This demonstrated that neither PLP1 nor ADRP plays an essential role in coronavirus RNA synthesis.

Project description:The cotranslational folding is recognized as a very cooperative process that occurs after the nearly completion of the polypeptide sequence of a domain. Here we investigated the challenges faced by polypeptide segments of a non-vectorial β-barrel fold. Besides the biological interest behind the SARS coronavirus non-structural protein 1 (nsp1, 117 amino acids), this study model has two structural features that motivated its use in this work: 1- its recombinant production is dependent on the temperature, with greater solubility when expressed at low temperatures. This is an indication of the cotranslational guidance to the native protein conformation. 2- Conversely, nsp1 has a six-stranded, mixed parallel/antiparallel β-barrel with intricate long-range interactions, indicating it will need the full-length protein to fold properly. We used non-denaturing purification conditions that allowed the characterization of polypeptide chains of different lengths, mimicking the landscape of the cotranslational fold of a β-barrel, and avoiding the major technical hindrances of working with the nascent polypeptide bound to the ribosome. Our results showed partially folded states formed as soon as the amino acids of the second β-strand were present (55 amino acids). These partially folded states are different based on the length of polypeptide chain. The native α-helix (amino acids 24-37) was identified as a transient structure (~20-30% propensity). We identified the presence of regular secondary structure after the fourth native β-strand is present (89 amino acids), in parallel to the collapse to a non-native 3D structure. Interestingly the polypeptide sequences of the native strands β2, β3 and β4 have characteristics of α-helices. Our comprehensive analyses support the idea that incomplete polypeptide chains, such as the ones of nascent proteins much earlier than the end of the translation, adopt an abundance of specific transient folds, instead of disordered conformations.

Project description:Coronaviruses (CoVs) initiate replication by translation of the positive-sense RNA genome into the replicase polyproteins connecting 16 nonstructural protein domains (nsp1-16), which are subsequently processed by viral proteases to yield mature nsp. For the betacoronavirus murine hepatitis virus (MHV), total inhibition of translation or proteolytic processing of replicase polyproteins results in rapid cessation of RNA synthesis. The nsp5-3CLpro (Mpro) processes nsps7-16, which assemble into functional replication-transcription complexes (RTCs), including the enzymatic nsp12-RdRp and nsp14-exoribonuclease (ExoN)/N7-methyltransferase. The nsp14-ExoN activity mediates RNA-dependent RNA proofreading, high-fidelity RNA synthesis, and replication. To date, the solved partial RTC structures, biochemistry, and models use or assume completely processed, mature nsp. Here, we demonstrate that in MHV, engineered deletion of the cleavage sites between nsp13-14 and nsp14-15 allowed recovery of replication-competent virus. Compared to wild-type (WT) MHV, the nsp13-14 and nsp14-15 cleavage deletion mutants demonstrated delayed replication kinetics, impaired genome production, altered abundance and patterns of recombination, and impaired competitive fitness. Further, the nsp13-14 and nsp14-15 mutant viruses demonstrated mutation frequencies that were significantly higher than with the WT. The results demonstrate that cleavage of nsp13-14 or nsp14-15 is not required for MHV viability and that functions of the RTC/nsp14-ExoN are impaired when assembled with noncleaved intermediates. These data will inform future genetic, structural, biochemical, and modeling studies of coronavirus RTCs and nsp 13, 14, and 15 and may reveal new approaches for inhibition or attenuation of CoV infection. IMPORTANCE Coronavirus replication requires proteolytic maturation of the nonstructural replicase proteins to form the replication-transcription complex. Coronavirus replication-transcription complex models assume mature subunits; however, mechanisms of coronavirus maturation and replicase complex formation have yet to be defined. Here, we show that for the coronavirus murine hepatitis virus, cleavage between the nonstructural replicase proteins nsp13-14 and nsp14-15 is not required for replication but does alter RNA synthesis and recombination. These results shed new light on the requirements for coronavirus maturation and replication-transcription complex assembly, and they may reveal novel therapeutic targets and strategies for attenuation.

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Membrane topology of murine coronavirus replicase nonstructural protein 3.

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Membrane topology of murine coronavirus replicase nonstructural protein 3.

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