The nucleocapsid domain of Gag is dispensable for actin incorporation into HIV-1 and for association of viral budding sites with cortical F-actin.
Actin and actin-binding proteins are incorporated into HIV-1 particles, and F-actin has been suggested to bind the NC domain in HIV-1 Gag. Furthermore, F-actin has been frequently observed in the vicinity of HIV-1 budding sites by cryo-electron tomography (cET). Filamentous structures emanating from viral buds and suggested to correspond to actin filaments have been observed by atomic force microscopy. To determine whether the NC domain of Gag is required for actin association with viral buds and for actin incorporation into HIV-1, we performed comparative analyses of virus-like particles (VLPs) obtained by expression of wild-type HIV-1 Gag or a Gag variant where the entire NC domain had been replaced by a dimerizing leucine zipper [Gag(LZ)]. The latter protein yielded efficient production of VLPs with near-wild-type assembly kinetics and size and exhibited a regular immature Gag lattice. Typical HIV-1 budding sites were detected by using cET in cells expressing either Gag or Gag(LZ), and no difference was observed regarding the association of buds with the F-actin network. Furthermore, actin was equally incorporated into wild-type HIV-1 and Gag- or Gag(LZ)-derived VLPs, with less actin per particle observed than had been reported previously. Incorporation appeared to correlate with the relative intracellular actin concentration, suggesting an uptake of cytosol rather than a specific recruitment of actin. Thus, the NC domain in HIV-1 Gag does not appear to have a role in actin recruitment or actin incorporation into HIV-1 particles. Importance: HIV-1 particles bud from the plasma membrane, which is lined by a network of actin filaments. Actin was found to interact with the nucleocapsid domain of the viral structural protein Gag and is incorporated in significant amounts into HIV-1 particles, suggesting that it may play an active role in virus release. Using electron microscopy techniques, we previously observed bundles of actin filaments near HIV-1 buds, often seemingly in contact with the Gag layer. Here, we show that this spatial association is observed independently of the proposed actin-binding domain of HIV-1. The absence of this domain also did not affect actin incorporation and had a minor effect on the viral assembly rate. Furthermore, actin was not enriched in the virus compared to the average levels in the respective producing cell. Our data argue against a specific recruitment of actin to HIV-1 budding sites by the nucleocapsid domain of Gag.
Department of Infectious Diseases, Virology, University Hospital Heidelberg, Heidelberg, Germany Oxford Particle Imaging Centre, Division of Structural Biology, Wellcome Trust Centre for Human Genetics, University of Oxford, Oxford, United Kingdom.
Sarah
S
Stauffer
Department of Infectious Diseases, Virology, University Hospital Heidelberg, Heidelberg, Germany.
Sheikh Abdul
SA
Rahman
Structural and Computational Biology Unit, European Molecular Biology Laboratory, Heidelberg, Germany Molecular Medicine Partnership Unit, Heidelberg, Germany.
Alex
A
de Marco
Department of Molecular Structural Biology, Max Planck Institute of Biochemistry, Martinsried, Germany.
Lars-Anders
LA
Carlson
Department of Infectious Diseases, Virology, University Hospital Heidelberg, Heidelberg, Germany.
Bärbel
B
Glass
Department of Infectious Diseases, Virology, University Hospital Heidelberg, Heidelberg, Germany.
Heike
H
Oberwinkler
Department of Infectious Diseases, Virology, University Hospital Heidelberg, Heidelberg, Germany.
Nikolas
N
Herold
Structural and Computational Biology Unit, European Molecular Biology Laboratory, Heidelberg, Germany Molecular Medicine Partnership Unit, Heidelberg, Germany.
John A G
JA
Briggs
Department of Infectious Diseases, Virology, University Hospital Heidelberg, Heidelberg, Germany.
Barbara
B
Müller
Oxford Particle Imaging Centre, Division of Structural Biology, Wellcome Trust Centre for Human Genetics, University of Oxford, Oxford, United Kingdom Department of Molecular Structural Biology, Max Planck Institute of Biochemistry, Martinsried, Germany kay@strubi.ox.ac.uk hans-georg.kraeusslich@med.uni-heidelberg.de.
Kay
K
Grünewald
Department of Infectious Diseases, Virology, University Hospital Heidelberg, Heidelberg, Germany Molecular Medicine Partnership Unit, Heidelberg, Germany kay@strubi.ox.ac.uk hans-georg.kraeusslich@med.uni-heidelberg.de.
Hans-Georg
HG
Kräusslich
D000199:Actins / Q000378:metabolism*; D002460:Cell Line; D015497:HIV-1 / Q000235:genetics / Q000502:physiology*; D054884:Host-Pathogen Interactions; D006801:Humans; D019251:Nucleocapsid / Q000235:genetics / Q000378:metabolism; D017434:Protein Structure, Tertiary; D019065:Virus Assembly; D057074:Virus Release; D054301:gag Gene Products, Human Immunodeficiency Virus / Q000235:genetics / Q000378:metabolism*
D000199:Actins; D054301:gag Gene Products, Human Immunodeficiency Virus