Cryo-electron Microscopy Structure of the Native Prototype Foamy Virus Glycoprotein and Virus Architecture.

Grégory Effantin,Leandro F Estrozi,Nick Aschman,Guy Schoehn,Winfried Weissenhorn,Dirk Lindemann,Nicole Stanke,Patricia Renesto,Hans-Georg Krausslich

doi:10.1371/journal.ppat.1005721

Abstract

Foamy viruses (FV) belong to the genus Spumavirus, which forms a distinct lineage in the Retroviridae family. Although the infection in natural hosts and zoonotic transmission to humans is asymptomatic, FVs can replicate well in human cells making it an attractive gene therapy vector candidate. Here we present cryo-electron microscopy and (cryo-)electron tomography ultrastructural data on purified prototype FV (PFV) and PFV infected cells. Mature PFV particles have a distinct morphology with a capsid of constant dimension as well as a less ordered shell of density between the capsid and the membrane likely formed by the Gag N-terminal domain and the cytoplasmic part of the Env leader peptide gp18LP. The viral membrane contains trimeric Env glycoproteins partly arranged in interlocked hexagonal assemblies. In situ 3D reconstruction by subtomogram averaging of wild type Env and of a Env gp48TM- gp80SU cleavage site mutant showed a similar spike architecture as well as stabilization of the hexagonal lattice by clear connections between lower densities of neighboring trimers. Cryo-EM was employed to obtain a 9 Å resolution map of the glycoprotein in its pre-fusion state, which revealed extensive trimer interactions by the receptor binding subunit gp80SU at the top of the spike and three central helices derived from the fusion protein subunit gp48TM. The lower part of Env, presumably composed of interlaced parts of gp48TM, gp80SU and gp18LP anchors the spike at the membrane. We propose that the gp48TM density continues into three central transmembrane helices, which interact with three outer transmembrane helices derived from gp18LP. Our ultrastructural data and 9 Å resolution glycoprotein structure provide important new insights into the molecular architecture of PFV and its distinct evolutionary relationship with other members of the Retroviridae.

Highlights

Spuma or foamy viruses (FV) are the only members of the Spumaretrovirinae subfamily of the Retroviridae
Foamy viruses (FVs), which belong to the retroviral genus Spumavirus, are endemic to non-human primates and can be transmitted to humans
We find that Prototype FV (PFV) contains a nucleocapsid of constant dimensions at its center, an intermediate shell of protein positioned between the core capsid and the viral membrane and glycoprotein that arranges into regular hexagonal lattices on the virus membrane

Summary

Introduction

Spuma or foamy viruses (FV) are the only members of the Spumaretrovirinae subfamily of the Retroviridae As such they share many similarities in their life cycle with the Orthoretrovirinae as well as some features with the more distant Hepadnaviridae [1]. PFV Gag is unusual in that it is not processed by the viral protease into canonical Matrix (MA), Capsid (CA) and Nucleocapsid (NC) domains, like other retroviral Gag polyproteins. PFV Gag lacks a membrane-binding domain and instead virus egress relies on a physical interaction between Gag and the Leader Peptide (LP) domain of the Envelope (Env) protein [9,10]. Capsids are transported to the secretory pathway either the ER or the Golgi or directly to the plasma membrane where Env LP interacts with the N-terminal Gag region [9,10,16]. Intracellular virions are most likely transported in vesicles for release at the plasma membrane (reviewed in [1])

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