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Proc Natl Acad Sci U S A. 2015 Feb 17;112(7):E649-56. doi: 10.1073/pnas.1420317112. Epub 2015 Feb 2.

CryoEM and mutagenesis reveal that the smallest capsid protein cements and stabilizes Kaposi's sarcoma-associated herpesvirus capsid.

Author information

1
Department of Microbiology, Immunology and Molecular Genetics, The California NanoSystems Institute, Bioengineering Program, and.
2
Department of Molecular and Medical Pharmacology, University of California, Los Angeles, CA 90095.
3
Department of Microbiology, Immunology and Molecular Genetics.
4
The California NanoSystems Institute, Bioengineering Program, and Department of Molecular and Medical Pharmacology, University of California, Los Angeles, CA 90095 rsun@mednet.ucla.edu hong.zhou@ucla.edu.
5
Department of Microbiology, Immunology and Molecular Genetics, The California NanoSystems Institute, Bioengineering Program, and rsun@mednet.ucla.edu hong.zhou@ucla.edu.

Abstract

With just one eighth the size of the major capsid protein (MCP), the smallest capsid protein (SCP) of human tumor herpesviruses--Kaposi's sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV)--is vital to capsid assembly, yet its mechanism of action is unknown. Here, by cryoEM of KSHV at 6-Å resolution, we show that SCP forms a crown on each hexon and uses a kinked helix to cross-link neighboring MCP subunits. SCP-null mutation decreased viral titer by 1,000 times and impaired but did not fully abolish capsid assembly, indicating an important but nonessential role of SCP. By truncating the C-terminal half of SCP and performing cryoEM reconstruction, we demonstrate that SCP's N-terminal half is responsible for the observed structure and function whereas the C-terminal half is flexible and dispensable. Serial truncations further highlight the critical importance of the N-terminal 10 aa, and cryoEM reconstruction of the one with six residues truncated localizes the N terminus of SCP in the cryoEM density map and enables us to construct a pseudoatomic model of SCP. Fitting of this SCP model and a homology model for the MCP upper domain into the cryoEM map reveals that SCP binds MCP largely via hydrophobic interactions and the kinked helix of SCP bridges over neighboring MCPs to form noncovalent cross-links. These data support a mechanistic model that tumor herpesvirus SCP reinforces the capsid for genome packaging, thus acting as a cementing protein similar to those found in many bacteriophages.

KEYWORDS:

Kaposi's sarcoma-associated herpesvirus; capsid assembly; capsid stabilization; cementing protein; smallest capsid protein

PMID:
25646489
PMCID:
PMC4343125
DOI:
10.1073/pnas.1420317112
[Indexed for MEDLINE]
Free PMC Article

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