RESEA r CH HIGHLIGHTS C E L L U L A R M I c ROBIOLOGY Virus plays dead Vaccinia virus is a large, complex, enveloped DNA virus that belongs to the Poxviridae family of viruses, which includes variola, the causative agent of smallpox. The infectious mature virus (MV) form of vaccinia has been shown to bind to actin-containing finger-like protrusions (filopodia) of the host cell and enter the cell in a pH- dependent manner. In a new study published in Science, Jason Mercer and Ari Helenius report that the MV form of vaccinia virus enters host cells using macropinocytosis and apoptotic mimicry. The authors prepared fluorescent MV particles and used live imaging to follow the entry of individual particles into host cells that expressed Two infected cells that are blebbing. Actin is shown in red and virus particles are shown in green. The blebs appear in cross-section as circles. Image kindly provided by Jason Mercer, Swiss Federal Institute of fluorescent actin. Virus particles that Technology (ETH), Zurich, Switzerland. bound to filopodia moved towards the cell body in an actin-dependent This further confirmed that these that viral PS might be analogous to manner. Once they reached the cell processes are linked. Sensitivity to cellular PS and triggers the uptake of body, membrane blebs formed at the Na+/H+ exchangers is also characteris- virus particles by mimicking apoptotic site of contact with virus, followed tic of macropinocytosis. Indeed, MVs bodies. Consistent with this, late-stage by the formation of further blebs co-internalized with fluid-phase cargo, vaccinia-infected cells were shown to along the entire cell body. The blebs but not clathrin-dependent cargo, in undergo apoptosis. eventually retracted, which coincided a macropinocytosis-type endocytic The clever use of apoptotic with virus entry. Inhibiting mem- process. mimicry and subsequent entry brane blebbing caused a large reduc- Apoptotic bodies are macropino- by macropinocytosis allows large tion in infection, which suggests that cytosed by phagocytes and other cell particles such as vaccinia virus blebbing is required for infection. types, and uptake of apoptotic debris to infect many different potential The serine/threonine kinase p21- is triggered by the presence of exposed host cells. And by posing as apop- activated kinase 1 (PAK1) is essential phosphatidylserine (PS) on the plasma totic bodies, MVs may also avoid for MV infection; indeed, knockdown membrane. The MV membrane is immune detection. of PAK1 reduced infection signifi- known to be enriched in PS, which cantly. MV infection is accompanied is required for infection. The authors Arianne Heinrichs by phosphorylation of residue Thr423 showed that PS is exposed on the Chief Editor, Nature Reviews Molecular of PAK1, which is known to be MV surface and that virus particles Cell Biology essential for macropinocytosis. PAK1 extracted with a detergent that knockdown studies also showed removes all lipids fail to induce bleb- ORIGINAL RESEARCH PAPER Mercer, J. & that PAK1 is required for events that bing and infection. Reconstitution of Helenius, A. Vaccinia virus uses macropinocytosis and apoptotic mimicry to enter host cells. occur prior to viral fusion, including the extracted virus particles with PS Science 320, 531–535 (2008) blebbing. Perturbance of several other restored infection. Because MVs are FURThER READInG Gruenberg, J. & van der factors, including the PAK1 activator comparable in size to apoptotic bodies Goot, G. Mechanisms of pathogen entry through + + the endosomal compartments. Nature Rev. Mol. GTPase Rac1 and Na /H exchangers, and the uptake mechanisms are the Cell Biol. 7, 495–504 (2006) inhibited both blebbing and infection. same, Mercer and Helenius concluded natUrE rEviEWs | MICROBIOLOGY voLUmE 6 | jUnE 2008 .