Structural highlights
Function
[POLS_AURAV] Capsid protein possesses a protease activity that results in its autocatalytic cleavage from the nascent structural protein. Following its self-cleavage, the capsid protein transiently associates with ribosomes, and within several minutes the protein binds to viral RNA and rapidly assembles into icosaedric core particles. The resulting nucleocapsid eventually associates with the cytoplasmic domain of E2 at the cell membrane, leading to budding and formation of mature virions. New virions attach to target cells, and after endocytosis their membrane fuses with the target cell membrane. This leads to the release of the nucleocapsid into the cytoplasm, followed by an uncoating event necessary for the genomic RNA to become accessible. The uncoating might be triggered by the interaction of capsid proteins with ribosomes. Binding of ribosomes would release the genomic RNA since the same region is genomic RNA-binding and ribosome-binding (By similarity). E3 protein's function is unknown (By similarity). E2 is responsible for viral attachment to target host cell, by binding to the cell receptor. Synthesized as a p62 precursor which is processed by furin at the cell membrane just before virion budding, giving rise to E2-E1 heterodimer. The p62-E1 heterodimer is stable, whereas E2-E1 is unstable and dissociate at low pH. p62 is processed at the last step, presumably to avoid E1 fusion activation before its final export to cell surface. E2 C-terminus contains a transitory transmembrane that would be disrupted by palmitoylation, resulting in reorientation of the C-terminal tail from lumenal to cytoplasmic side. This step is critical since E2 C-terminus is involved in budding by interacting with capsid proteins. This release of E2 C-terminus in cytoplasm occurs lately in protein export, and precludes premature assembly of particles at the endoplasmic reticulum membrane (By similarity). 6K is a constitutive membrane protein involved in virus glycoprotein processing, cell permeabilization, and the budding of viral particles. Disrupts the calcium homeostasis of the cell, probably at the endoplasmic reticulum level. This leads to cytoplasmic calcium elevation. Because of its lipophilic properties, the 6K protein is postulated to influence the selection of lipids that interact with the transmembrane domains of the glycoproteins, which, in turn, affects the deformability of the bilayer required for the extreme curvature that occurs as budding proceeds. Present in low amount in virions, about 3% compared to viral glycoproteins (By similarity). E1 is a class II viral fusion protein. Fusion activity is inactive as long as E1 is bound to E2 in mature virion. After virus attachment to target cell and endocytosis, acidification of the endosome would induce dissociation of E1/E2 heterodimer and concomitant trimerization of the E1 subunits. This E1 trimer is fusion active, and promotes release of viral nucleocapsid in cytoplasm after endosome and viral membrane fusion. Efficient fusion requires the presence of cholesterol and sphingolipid in the target membrane (By similarity).
Publication Abstract from PubMed
The alphavirus capsid protein (CP) is a serine protease that possesses cis-proteolytic activity essential for its release from the nascent structural polyprotein. The released CP further participates in viral genome encapsidation and nucleocapsid core formation, followed by its attachment to glycoproteins and virus budding. Thus, protease activity of the alphavirus capsid is a potential antialphaviral target to arrest capsid release, maturation, and structural polyprotein processing. However, the discovery of capsid protease inhibitors has been hampered due to the lack of a suitable screening assay and of the crystal structure in its active form. Here, we report the development of a trans-proteolytic activity assay for Aura virus capsid protease (AVCP) based on fluorescence resonance energy transfer (FRET) for screening protease inhibitors. Kinetic parameters using fluorogenic peptide substrates were estimated, and the Km value was found to be 2.63 +/- 0.62 muM while the kcat/Km value was 4.97 x 10(4) M(-1) min(-1). Also, the crystal structure of the trans-active form of AVCP has been determined to 1.81-A resolution. Structural comparisons of the active form with the crystal structures of available substrate-bound mutant and inactive blocked forms of the capsid protease identify conformational changes in the active site, the oxyanion hole, and the substrate specificity pocket residues, which could be critical for rational drug design. IMPORTANCE The alphavirus capsid protease is an attractive antiviral therapeutic target. In this study, we have described the formerly unappreciated trans-proteolytic activity of the enzyme and for the first time have developed a FRET-based protease assay for screening capsid protease inhibitors. Our structural studies unveil the structural features of the trans-active protease, which has been previously proposed to exist in the natively unfolded form (M. Morillas, H. Eberl, F. H. Allain, R. Glockshuber, and E. Kuennemann, J. Mol. Biol. 376:721-735, 2008, doi:http://dx.doi.org/10.1016/j.jmb.2007.11.055). The different enzymatic forms have been structurally compared to reveal conformational variations in the active and substrate binding sites. The flexible active-site residue Ser218, the disordered C-terminal residues after His261, and the presence of a water molecule in the oxyanion hole of AVCPDelta2 (AVCP with a deletion of the last two residues at the C terminus) reveal the effect of the C-terminal Trp267 deletion on enzyme structure. New structural data reported in this study along with the fluorogenic assay will be useful in substrate specificity characterization, high-throughput protease inhibitor screening, and structure-based development of antiviral drugs.
trans-Protease Activity and Structural Insights into the Active Form of the Alphavirus Capsid Protease.,Aggarwal M, Dhindwal S, Kumar P, Kuhn RJ, Tomar S J Virol. 2014 Nov 1;88(21):12242-53. doi: 10.1128/JVI.01692-14. Epub 2014 Aug 6. PMID:25100849[1]
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.
References
- ↑ Aggarwal M, Dhindwal S, Kumar P, Kuhn RJ, Tomar S. trans-Protease Activity and Structural Insights into the Active Form of the Alphavirus Capsid Protease. J Virol. 2014 Nov 1;88(21):12242-53. doi: 10.1128/JVI.01692-14. Epub 2014 Aug 6. PMID:25100849 doi:http://dx.doi.org/10.1128/JVI.01692-14
