Structural Polyprotein (C) O'nyong-Nyong Virus Recombinant Protein

Function: 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, 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.

Catalytic activity: Autocatalytic release of the core protein from the N-terminus of the togavirus structural polyprotein by hydrolysis of a -Trp-|-Ser- bond.

Subunit structure: p62 and E1 form a heterodimer shortly after synthesis. Processing of p62 into E2 and E3 results in a heterodimer of E2 and E1. Spike at virion surface are constituted of three E2-E1 heterodimers. After target cell attachment and endocytosis, E1 change conformation to form homotrimers

By similarity.

Subcellular location: Capsid protein: Virion

By similarity. Host cytoplasm

By similarity. p62: Virion membrane; Single-pass type I membrane protein

By similarity. Host cell membrane; Single-pass type I membrane protein

By similarity. E2 envelope glycoprotein: Virion membrane; Single-pass type I membrane protein

By similarity. Host cell membrane; Single-pass type I membrane protein

By similarity. E1 envelope glycoprotein: Virion membrane; Single-pass type I membrane protein

By similarity. Host cell membrane; Single-pass type I membrane protein

By similarity. 6K protein: Host cell membrane; Multi-pass membrane protein

By similarity. Virion membrane; Multi-pass membrane protein

By similarity.

Post-translational modification: Specific enzymatic cleavages in vivo yield mature proteins. Capsid protein is auto-cleaved during polyprotein translation, unmasking p62 signal peptide. The remaining polyprotein is then targeted to the endoplasmic reticulum, where host signal peptidase cleaves it into p62, 6K and E1 proteins. p62 is further processed to mature E3 and E2 by host furin in trans-Golgi vesicle

By similarity.E2 and 6K are palmitoylated via thioester bonds

By similarity.

Miscellaneous: Structural polyprotein is translated from a subgenomic RNA synthesized during togavirus replication.

Sequence similarities: Contains 1 peptidase S3 domain.