Simian Virus 40 (SV40) is a non-enveloped double-stranded DNA virus of the papova virus family. It is a remarkably simple virus, just encoding three structural proteins and two non-structural proteins, and takes histones from the host cell to condense its genome into a ‘mini-chromosome’. SV40 has served as a paradigm model in the early days of molecular biology. Many mammalian expression vectors are based on SV40 sequences, and it has been much-used in studies of DNA replication and of cellular transformation by Large T antigen. In the past 15 years, it has also served as an important model in the field of endocytosis and membrane trafficking, as it was the first virus discovered to utilise a clathrin-independent endocytic uptake route involving caveolae and other types of membrane invaginations . The organelles in which the virus is internalised are often tubular, move along microtubules, and serve as transport carriers to the endoplasmic reticulum. Since SV40 can be easily purified and individual virus particles can be labelled with >100 fluorophores without compromising infectivity, it was used in early studies employing single-particle tracking in time-lapse imaging.
Recently, we have focussed on the complexity of cell surface rearrangements and intracellular signalling induced by SV40 upon binding to its host cell . One striking finding is that binding of SV40 leads to rapid and complete dephosphorylation of Ezrin, which couples the actin cytoskeleton to the cytosolic surface of the plasma membrane in its phosphorylated form. Delineating the molecular signal transduction events responsible for this revealed that SV40 induces signal transduction by binding to integrins, and not via glycosphingolipids. Thus, SV40 utilises classical forms of signal transduction via transmembrane proteins. This leads to the activation of AKT via integrin-linked kinase, PI-3 kinase and phosphoinositide-dependent kinase 1 (PDK1), and to the inactivation of RhoA via GRAF1. The latter leads to a loss of phosphorylated Ezrin, allowing actin to uncouple from the membrane. This actin-membrane uncoupling leads to an increase in caveolae dynamics and membrane blebbing.
- What is the role of AKT activation during SV40 infection of host cells? That this activation lasts for at least 8 hours upon SV40 binding to cells suggests that it may act as a survival signal to overrule potential apoptosis-inducing signals when SV40 exits the endoplasmic reticulum.
- What is the role of caveolae in SV40-induced membrane blebbing? Do blebs form from a membrane reservoir provided by caveolae, and would this membrane be internalized by endocytosis upon bleb retraction, thereby internalizing SV40 particles?
- Could integrin-mediated signalling, induced by SV40 but for instance also by membrane tension, lead to disassembly of the caveolar coat? What are the molecular players involved, and could phosphorylation of coat subunits play a role?
Current lab members involved:
Some relevant publications from the lab: