Cyclophilins Facilitate Dissociation of the Human Papillomavirus Type 16 Capsid Protein L1 from the L2/DNA Complex following Virus Entry

Abstract

<jats:p>Human papillomaviruses (HPV) are composed of the major and minor capsid proteins, L1 and L2, that encapsidate a chromatinized, circular double-stranded DNA genome. At the outset of infection, the interaction of HPV type 16 (HPV16) (pseudo)virions with heparan sulfate proteoglycans triggers a conformational change in L2 that is facilitated by the host cell chaperone cyclophilin B (CyPB). This conformational change results in exposure of the L2 N terminus, which is required for infectious internalization. Following internalization, L2 facilitates egress of the viral genome from acidified endosomes, and the L2/DNA complex accumulates at PML nuclear bodies. We recently described a mutant virus that bypasses the requirement for cell surface CyPB but remains sensitive to cyclosporine for infection, indicating an additional role for CyP following endocytic uptake of virions. We now report that the L1 protein dissociates from the L2/DNA complex following infectious internalization. Inhibition and small interfering RNA (siRNA)-mediated knockdown of CyPs blocked dissociation of L1 from the L2/DNA complex.<jats:italic>In vitro</jats:italic>, purified CyPs facilitated the dissociation of L1 pentamers from recombinant HPV11 L1/L2 complexes in a pH-dependent manner. Furthermore, CyPs released L1 capsomeres from partially disassembled HPV16 pseudovirions at slightly acidic pH. Taken together, these data suggest that CyPs mediate the dissociation of HPV L1 and L2 capsid proteins following acidification of endocytic vesicles.</jats:p>

Genome-wide siRNA screen identifies the retromer as a cellular entry factor for human papillomavirus

“…Other laboratories showed that L1 dissociates from L2 during HPV entry and is sorted to the lysosome for degradation (39). We found that some L1, like L2, remains physically associated with the retromer and traffics to a Golgi-like compartment.…”

Incoming human papillomavirus type 16 genome resides in a vesicular compartment throughout mitosis

“…For example, herpesviruses and adenoviruses both egress into the cytoplasm, yet protect their viral DNA by keeping it encased in viral capsids until directly transferring it into the nucleus through the nuclear pore complex. In contrast, the capsid is unlikely to protect papillomavirus (PV) genomes while traversing the cytoplasm, because the major capsid protein is lost in the endocytic compartment (2).…”

“…Following primary attachment and internalization, acidification of early endosomes triggers capsid disassembly (7-22). Host cell cyclophilins release the majority of L1 from the L2 protein, which remains in complex with the viral genome (2,23,24). A large portion of the L2 protein translocates across the endocytic membrane to engage factors, including the retromer complex, dynein, sorting nexins, and rab GTPases, that mediate transport to the trans-Golgi network (TGN) (25)(26)(27)(28)(29)(30)(31)(32)(33)).…”

Antiviral drug discovery: broad-spectrum drugs from nature

“…CypA interacts with viral proteins supporting viral replication 69 . CypA inhibitors such as cyclosporine A (CsA) have been shown to inhibit the replication of HIV, HCV, influenza virus, CoV, HBV, HSV, human cytomegalovirus (HCMV), VSV, vaccinia virus (VV) and human papillomavirus (HPV) [105][106][107][108][109][110][111][112][113][114] . Alisporivir (Debio-025) and SCY-635, both CsA analogues, have shown antiviral activity against HCV in vivo and are currently in combination with other anti-HCV compounds in various clinical trials 115,116 .…”