Uncover the unique pathway for monkey virus SV40 to sneak into the nucleus

The final and decisive step in infection of most DNA viruses is entry into the nucleus
.
Polyomavirus is a DNA tumor virus that causes a variety of debilitating human diseases, but how it reaches the nucleus and causes infection is a mystery
.

A team of researchers at the University of Michigan School of Medicine conducted an in-depth study of how the polyomavirus SV40 enters the nuclear membrane and found that two major nuclear membrane protein complexes – LINC and NPC – work synergistically to drive the prototype polyomavirus SV40 into the nucleus
.

SV40 viral particles bind to receptors on the plasma membrane of the target host cell, under receptor-mediated endocytosis, the virus is endocytosed and sorted along the complex endometrial network and delivered to endosomes, the virus SV40 bypasses the Golgi apparatus and is transferred directly to the endoplasmic reticulum (ER) through a mechanism at the endosome-ER membrane contact site, and the hydrophobic VP2 and VP3 internal proteins of SV40 are exposed, producing hydrophobic particles that can integrate at ER-foci and penetrate ER to reach the cytoplasm
。 Once SV40 escapes into the cytoplasm, SV40 targets the nuclear membrane by directly binding to the nuclear skeleton and Nesprin-2 of the cytoskeletal junction (LINC) nuclear membrane complex; The NUP188 subunit of the nuclear pore complex (NPC) interacts with Nesprin-2, and this physical proximity enables NPCs to capture SV40 released from Nesprin-2, enabling nuclear pore channels to facilitate nuclear translocation
of the virus.
Strikingly, SV40 breaks down as it enters the nucleus, producing the viral genome-VP1-VP3 subcomplex, which effectively passes through NPCs into the nucleus and finally reaches an intracellular destination
that supports viral genome replication.

Studies have shown that knocking out Nesprin-2 can block the infectious ability of SV40, SV40 directly binds to Nesprin-2 during entry, SV40 containing VP1 and VP3 is transported to the nucleus after viral pentameric dissociation, NPC subunit Nup188 binds to Nesprin-2 and promotes SV40 entry into the nucleus
.

The results revealed how two major nuclear membrane protein complexes can be used to facilitate viral targeting and translocation into the nucleus
.
Elucidating the critical steps of polyomavirus entry into the nucleus may lead to new strategies to combat diseases caused by polyomaviruses and provide a new understanding
of nuclear transport mechanisms.
Studying how viral infections have evolved in mammals can even answer fundamental questions
about human health.

"SV40 is used as a tool to work on understanding how viruses cause cancer in humans," said Chelsey C.
Spriggs, Ph.
D.
, assistant professor of cell and developmental biology, microbiology and immunology at the University of Michigan School of Medicine and research assistant professor at the University of Michigan's Institute for Life Sciences, and first author
of the study.
Several viruses have been linked to human cancer, including human papillomavirus, Kaposisarcoma-associated herpesvirus, and Epstein-Barr virus
.

The team hopes to gain a fuller understanding of how this infection process occurs
inside cells.
An early study by Spriggs found that SV40 travels from the cell surface, through endosomes, endoplasmic reticulum, and then to the cytoplasm, where it is partially broken down
.
The latest research sheds light on the final and most important step of infection, which is entry into the nucleus
.

Spriggs explained that the virus itself is larger than the entrance into the nucleus, known as the nuclear pore complex
.
The nuclear pore complex is a major port on the cell membrane of the nucleus, regulating the transport
of proteins, RNA, and other cellular cargo from the nucleus into the cytoplasm and back.
Many viruses use this pathway to sneak into the nucleus
.

The new study found that SV40 uses the nuclear pore complex and another protein complex called LINC (LINC connects the inner and outer membranes of the cell nucleus) to first break itself down into a smaller package
made up of two proteins and viral genomes.
Unlike many other viruses that grasp the protrusions protruding from the nuclear pore complex, SV40 interacts
with the LINC before entering the LINC.
This difference may underlie
SV40's carcinogenic ability.
Further research into how SV40 utilizes LINC and the nuclear pore complex could even help scientists understand how these two important cell membrane complexes interact.

"The virus uses a lot of the same pathways that are destroyed in cancer and other diseases," Spriggs said
.
"Studying them helps to understand human biology
.
" Spriggs recently opened its own independent research lab at the University of Michigan to study the entry mechanism
of human oncoviruses.