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Budding viral hijackers co-opt the endocytic machinery to make a getaway
© BioMed Central Ltd 2003
- Published: 19 December 2003
A retroviral Gag protein interacts with a cellular protein involved in forming endocytic vesicles in a manner that affects the production of virus particles. These findings help define the suite of host-cell components that are usurped by the virus to help it propagate.
- Human Immunodeficiency Virus
- Simian Immunodeficiency Virus
- Internal Ribosome Entry Site
- Rous Sarcoma Virus
- Endocytic Vesicle
Viruses are champions when it comes to commandeering the normal biological processes of their hosts. In the case of viruses with small genomes, such as the retroviral human immunodeficiency virus (HIV) and influenza, the limited coding capacity of the viral genome forces the virus to use many host cell factors to extend its capabilities during entry into, replication within, and budding from the cells of its host. There is currently a lot of interest in understanding how retroviruses interact with their hosts. Work in this area not only helps us understand how viruses replicate, but also sheds light on normal cellular processes.
A great deal of attention has been given to how viruses latch onto cell-surface receptors and hijack other components to enter and replicate in the host cells, but much less is known about how they usurp the cellular machinery to orchestrate their exodus. Recently, a new model of virus budding has come onto the scene. It posits that enveloped RNA viruses bud by appropriating the endocytic cellular machinery that is normally used to create vesicles inside the cell: the formation of vesicles and the budding of a virus are topologically the same process, but the reverse of each other. This model has many implications for cell biology and viral pathogenesis, and virologists are now busy uncovering how viruses use the machinery of endocytosis to their benefit.
All retroviruses have three key genes, encoding proteins called Gag, Env and Pol. Gag is the structural protein that makes up the viral core and drives viral assembly and release. Gag is a polyprotein and is organized into four distinct regions: the matrix (MA) domain, which is closely associated with the plasma membrane and implicated in budding functions; the capsid (CA), which condenses to form ordered core particles that make up the internal shell of the virus; the nucleocapsid (NC), an RNA-binding protein; and a cleavage product whose name, as well as function, varies depending on the virus (for example, for HIV it is called p6 and for Mo-MuLV it is p12).
To identify further which cellular factors are recruited by Mo-MuLV during the production and budding of virus particles, Wang et al.  used a yeast two-hybrid assay of a mouse T-lymphoma cDNA library using Gag as bait and identified endophilin 2 as a Gag-binding partner. A second yeast two-hybrid screen showed that endophilin 2 interacts specifically with the MA portion of Gag. In vitro binding assays further confirmed the endophilin-Gag association.
Members of the endophilin family of proteins are involved in endocytic vesicle formation. Endophilin 2 is one of three members of the subgroup endophilin A and is a regulatory component of the machinery involved in clathrin-mediated endocytosis. These proteins are known to promote membrane curvature and bending and are involved in the vesicular trafficking events of endocytosis .
Wesley Sundquist, in the Department of Biochemistry at the University of Utah in Salt Lake City, USA, agrees and is quite excited about Goff's discovery. "This is of particular interest because it indicates that proteins that normally function primarily in the process of endocytosis or endosomal trafficking also participate in Mo-MuLV replication," he says.
Next, Wang et al.  set out to determine whether endophilins were incorporated into virus particles. They did experiments in which viral particles from cell lysates were harvested using a sucrose density gradient and proteins in these virions were detected and analyzed by western (immuno-) blotting. They found that both endogenous and exogenous endophilin 2 (tagged with the influenza hemagglutinin, HA) is packaged into Mo-MuLV particles. Additional tests confirmed that endophilin 2 was indeed present inside the virion and not simply a contaminant in the purified virions or attached on the outer surface of the particle.
To ensure that endophilin 2 is not merely incorporated into virion particles because of its proximity to the plasma membrane, a clever experiment was carried out whereby western blots were probed with antibodies specific for various other endocytic proteins. The virologists found that along with endophilin 2, Mo-MuLV particles contained the prevalent cellular proteins α-actin and clathrin, but not dynamin 2, the major endocytic partner of endophilin. Hence, it appears that the association of endophilin 2 with Gag is deliberate and is likely to play a specific role in virion production.
- Wang MQ, Kim W, Gao G, Torrey TA, Morse HC, De Camilli P, Goff S: Endophilins interact with Moloney murine leukemia virus Gag and modulate virion production. J Biol. 2003, 3: 4-10.1186/1475-4924-3-4.PubMedPubMed CentralView ArticleGoogle Scholar
- Reutens AT, Begley CG: Endophilin-1: a multifunctional protein. Int J Biochem Cell Biol. 2002, 34: 1173-1177. 10.1016/S1357-2725(02)00063-8.PubMedView ArticleGoogle Scholar
- Basyuk E, Galli T, Mougel M, Blanchard JM, Sitbon M, Bertrand E: Retroviral genomic RNAs are transported to the plasma membrane by endosomal vesicles. Dev Cell. 2003, 5: 161-174.PubMedView ArticleGoogle Scholar
- Strack B, Calistri A, Craig S, Popova E, Göttlinger HG: AIP1/ALIX is a binding partner for HIV-1 p6 and EIAV p9 functioning in virus budding. Cell. 2003, 114: 689-699.PubMedView ArticleGoogle Scholar
- von Schwedler UK, Stuchell M, Müller B, Ward DM, Chung HY, Morita E, Wang HE, Davis T, He GP, Cimbora DM: The protein network of HIV budding. Cell. 2003, 114: 701-713.PubMedView ArticleGoogle Scholar
- Martin-Serrano J, Yaravoy A, Perez-Caballero D, Bieniasz PD: Divergent retroviral late-budding domains recruit vacuolar protein sorting factors by using alternative adaptor proteins. Proc Natl Acad Sci USA. 2003, 100: 12414-12419. 10.1073/pnas.2133846100.PubMedPubMed CentralView ArticleGoogle Scholar
- Pornillos O, Garrus JE, Sundquist WI: Mechanisms of enveloped RNA virus budding. Trends Cell Biol. 2002, 12: 569-579. 10.1016/S0962-8924(02)02402-9.PubMedView ArticleGoogle Scholar
- Pelchen-Matthews A, Kramer B, Marsh M: Infectious HIV-1 assembles in late endosomes in primary macrophages. J Cell Biol. 2003, 162: 443-455. 10.1083/jcb.200304008.PubMedPubMed CentralView ArticleGoogle Scholar