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Retrovirology. 2016 Aug 22;13(1):57. doi: 10.1186/s12977-016-0291-8.

Mutagenesis of N-terminal residues of feline foamy virus Gag reveals entirely distinct functions during capsid formation, particle assembly, Gag processing and budding.

Author information

1
Department of Molecular Diagnostics of Oncogenic Infections, Research Program Infection and Cancer, German Cancer Research Center (DKFZ), Im Neuenheimer Feld 242, 69120, Heidelberg, Germany.
2
Center for Cancer Research, National Cancer Institute, Frederick, MD, USA.
3
CellNetworks, Bioquant, University of Heidelberg, Im Neuenheimer Feld 267, 69120, Heidelberg, Germany.
4
Department of Oncology, University of Oxford, Oxford, UK.
5
Biology Department, East China Normal University, Shanghai, China.
6
Biochemie Zentrum Heidelberg (BZH), Im Neuenheimer Feld 328, 69120, Heidelberg, Germany.
7
Department of Molecular Diagnostics of Oncogenic Infections, Research Program Infection and Cancer, German Cancer Research Center (DKFZ), Im Neuenheimer Feld 242, 69120, Heidelberg, Germany. m.loechelt@dkfz.de.

Abstract

BACKGROUND:

Foamy viruses (FVs) of the Spumaretrovirinae subfamily are distinct retroviruses, with many features of their molecular biology and replication strategy clearly different from those of the Orthoretroviruses, such as human immunodeficiency, murine leukemia, and human T cell lymphotropic viruses. The FV Gag N-terminal region is responsible for capsid formation and particle budding via interaction with Env. However, the critical residues or motifs in this region and their functional interaction are currently ill-defined, especially in non-primate FVs.

RESULTS:

Mutagenesis of N-terminal Gag residues of feline FV (FFV) reveals key residues essential for either capsid assembly and/or viral budding via interaction with the FFV Env leader protein (Elp). In an in vitro Gag-Elp interaction screen, Gag mutations abolishing particle assembly also interfered with Elp binding, indicating that Gag assembly is a prerequisite for this highly specific interaction. Gradient sedimentation analyses of cytosolic proteins indicate that wild-type Gag is mostly assembled into virus capsids. Moreover, proteolytic processing of Gag correlates with capsid assembly and is mostly, if not completely, independent from particle budding. In addition, Gag processing correlates with the presence of packaging-competent FFV genomic RNA suggesting that Pol encapsidation via genomic RNA is a prerequisite for Gag processing. Though an appended heterogeneous myristoylation signal rescues Gag particle budding of mutants unable to form capsids or defective in interacting with Elp, it fails to generate infectious particles that co-package Pol, as evidenced by a lack of Gag processing.

CONCLUSIONS:

Changes in proteolytic Gag processing, intracellular capsid assembly, particle budding and infectivity of defined N-terminal Gag mutants highlight their essential, distinct and only partially overlapping roles during viral assembly and budding. Discussion of these findings will be based on a recent model developed for Gag-Elp interactions in prototype FV.

KEYWORDS:

Assembly; Capsid formation; Feline foamy virus; Foamy virus; Gag; Genome packaging; Mutagenesis; Particle budding and maturation; Transmission

PMID:
27549192
PMCID:
PMC4994201
DOI:
10.1186/s12977-016-0291-8
[Indexed for MEDLINE]
Free PMC Article

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