The Alphavirus Exit Pathway: What We Know and What We Wish We Knew

Abstract

Alphaviruses are enveloped positive sense RNA viruses and include serious human pathogens, such as the encephalitic alphaviruses and Chikungunya virus. Alphaviruses are transmitted to humans primarily by mosquito vectors and include species that are classified as emerging pathogens. Alphaviruses assemble highly organized, spherical particles that bud from the plasma membrane. In this review, we discuss what is known about the alphavirus exit pathway during a cellular infection. We describe the viral protein interactions that are critical for virus assembly/budding and the host factors that are involved, and we highlight the recent discovery of cell-to-cell transmission of alphavirus particles via intercellular extensions. Lastly, we discuss outstanding questions in the alphavirus exit pathway that may provide important avenues for future research.

Keywords: alphavirus; assembly; budding; cell-to-cell transmission; intercellular extensions.

Figure 1

Schematic diagrams of alphavirus structural proteins. (A) Cartoon illustration of an alphavirus particle. E2 (blue) and E1 (orange) assemble into trimers of heterodimers embedded in the viral membrane bilayer (black). E2 directly interacts with capsid protein (green), and capsid protein assembles with the genomic RNA (enclosed dark gray sphere and lines) to form the viral nucleocapsid. Components not to scale; (B) cartoon illustration of the major (left) and minor (right) mature structural protein translation products. E3 is shown in grey, 6K in purple, transframe (TF) in purple and pink, and the other proteins are colored as in (A). Proteins not to scale.

Figure 2

Cartoon illustration of the p62-E1 heterodimer. E3 is shown in light gray. E2 domain A is shown in cyan, domain B in green, domain C in pink, and the B-ribbon connector in purple. E2 subdomain D is not shown. E1 domain I is shown in red, domain II in yellow, and domain III in blue. E1's fusion loop is shown as an orange star. E2 and E1 transmembrane helices are shown as rectangles colored according to their respective preceding domains. The lipid bilayer is depicted as dark gray lines.

Figure 3

Schematic model of specialized budding sites of an alphavirus infected cell (gray). Alphaviruses assemble and bud from localized patches at the plasma membrane of the cell body, where host proteins are excluded and Cp/NC serves as a scaffold to accumulate E2/E1 glycoproteins. Dramatic cytoskeletal remodeling occurs during infection, and short filopodia-like extensions (top) and long intercellular extensions (middle) are observed. Short extensions contain F-actin (yellow) only, while intercellular extensions contain F-actin and tubulin (blue) structures. Nascent virus particles bud along the short extension, which may facilitate dissemination from the infected cell. An intercellular extension projects from the infected cell and makes preferential contact with an uninfected cell (green). The contact site forms a stable, flattened tip on the target cell plasma membrane (right), producing a protected pocket where nascent particles bud and are subsequently internalized by the uninfected cell. Thus, intercellular extensions are close-ended membrane bridges that mediate cell-to-cell transmission. Host determinants involved in modulating the cytoskeleton, forming/stabilizing extensions, and promoting cell-to-cell virus transmission are currently unknown.