The cell biology of receptor-mediated virus entry

Grove, Joe; Marsh, Mark

In contrast, macropinocytosis, which usually mediates the uptake of large volumes of extracellular fluid and bulky cargo such as apoptotic bodies (Mercer and Helenius, 2008; Mercer et al., 2010b) , involves actin remodeling mediated by Rac-1 GTPase and its effector p21-activated kinase 1 (Pak-1), leading to the extension of membrane ruffles and blebs from the cell surface. These large membrane protrusions can fold/drop back on themselves, enclosing extracellular material (Swanson, 2008) . Macropinocytosis can occur constitutively in professional phagocytes such as dendritic cells but can be induced in other cell types by activation of tyrosine kinases such as EGFR (Swanson, 2008) . Kaposi's sarcoma-associated herpesvirus, adenovirus (2 and 3), echovirus 1, Ebola virus, and Vaccinia, the prototype poxvirus, are internalized via macropinocytosis (Amstutz et al., 2008; Liberali et al., 2008; Mercer and Helenius, 2008; Raghu et al., 2009; Mercer et al., 2010a; Nanbo et al., 2010; Saeed et al., 2010; Valiya Veettil et al., 2010; Schmidt et al., 2011) . Poxviruses have the unusual characteristic of producing two forms of infectious particle. Mature virions are brick-shaped particles that form in the cytoplasm of infected cells and possess a single-bilayer membrane. These particles are released when infected cells lyse. A second form, the so-called extracellular virus, is a mature virion that undergoes further envelopment by wrapping in membrane cisternae derived from the TGN or endosomes. These particles are secreted before cell lysis and have two membranes (Roberts and Smith, 2008) . During entry, mature virus particles attach to preexisting filopodia and migrate toward the cell body, where they induce strainspecific atypical macropinocytosis via membrane blebbing or filopodial extension (Mercer et al., 2010a) . The cellular receptors for Vaccinia virus are unknown, and it remains unclear how these processes are initiated. However, attachment of mature virions activates EGFR, Rho-GTPases, and actin remodeling (Mercer and Helenius, 2008; Mercer et al., 2010a) . Phosphatidylserine associated with the mature virion membrane has been proposed to contribute to Vaccinia virus-induced macropinocytosis in a process mimicking the uptake of apoptotic cells (Mercer and Helenius, 2008) , though another study disputes this (Laliberte and Moss, 2009) . Recent work suggests that entry of the extracellular virion also involves macropinocytosis, though phosphatidylserine is not involved, and entry is not affected by exogenous addition of the phosphatidylserine-binding protein annexin 5 (Schmidt et al., 2011) .