Mechanism of Membrane Penetration by Nonenveloped Polyomavirus and Papillomavirus

[摘要] Membrane penetration represents a critical step during virus infection. As nonenveloped viruses lack a surrounding lipid bilayer, they are unable to penetrate host membranes by a membrane fusion mechanism. Consequently, nonenveloped viruses must devise alternative strategies to enter the host cell. In the case of polyomavirus SV40 and human papillomavirus (HPV), these two nonenveloped DNA tumor viruses must hijack selective host factors in order to promote their membrane penetration. Upon endocytosis, SV40 traffics through the endosomal pathway to reach the endoplasmic reticulum (ER). Here the virion is inserted into the ER membrane and is extracted into the cytosol by the cytosolic extraction machinery composed of the Hsc70, SGTA, and Hsp105 chaperones. From the cytosol, the virus is transported into the nucleus to cause infection. My thesis work unambiguously identified Hsc70 as a critical component of the cytosolic extraction machinery that ejects SV40 from the ER into the cytosol, clarified the structure-function relationship of SGTA during this process, and unveiled SGTA’s ability to negatively regulate Hsc70’s action during this the ER-to-cytosol membrane transport step. Moreover, my studies revealed that the Bag2 nucleotide exchange factor (NEF) is a new component of the cytosolic extraction machinery. In this context, Bag2 stimulates SV40 release from Hsc70, promoting successful arrival of SV40 to the cytosol, leading to infection. Collectively, my findings identify a novel component of a cytosolic extraction machinery essential duringmembrane penetration of a nonenveloped virus, and provide further mechanistic insights into this process.Similar to SV40, HPV membrane penetration requires host factors that are poorly characterized. After initial entry, HPV reaches the endosome, where the viral L2 minor capsid protein is inserted into the endosomal membrane. Membrane insertion of L2 is a decisive event because this step recruits the cytosolic sorting retromer complex to endosome-localized HPV, which in turn directs the virus to the Golgi apparatus. From this compartment, the virus enters the nucleus during mitosis where viral DNA is replicated. Through classic biochemical analyses, we recently reported that the transmembrane protease gamma secretase acts as a novel chaperone that binds to and inserts L2 into the endosomal membrane, an essential HPV infection step. In this thesis, we now identify the gamma secretase-binding partner delta-catenin/p120 as a new host factor that interacts with L2 and promotes HPV infection. Our analysis further suggests a model in which p120 engages HPV early in infection, delivering the virus to gamma secretase so that HPV can properly insert into the endosome membrane. In sum, my results provide fundamental insights into the infectious entry pathway of the nonenveloped SV40 and HPV by illuminating their membrane penetration mechanism.

[发布机构] University of Michigan

[效力级别] Microbiology and Immunology

[关键词] Nonenveloped virus membrane penetration;Microbiology and Immunology;Science;Microbiology & Immunology