Major capsid reinforcement by a minor protein in herpesviruses and phage.

Udom Sae-Ueng,Alex Evilevitch,Fred L Homa,Jamie B Huffman,Ting Liu,Carlos Enrique Catalano

doi:10.1093/nar/gku634

Abstract

Herpes simplex type 1 virus (HSV-1) and bacteriophage λ capsids undergo considerable structural changes during self-assembly and DNA packaging. The initial steps of viral capsid self-assembly require weak, non-covalent interactions between the capsid subunits to ensure free energy minimization and error-free assembly. In the final stages of DNA packaging, however, the internal genome pressure dramatically increases, requiring significant capsid strength to withstand high internal genome pressures of tens of atmospheres. Our data reveal that the loosely formed capsid structure is reinforced post-assembly by the minor capsid protein UL25 in HSV-1 and gpD in bacteriophage λ. Using atomic force microscopy nano-indentation analysis, we show that the capsid becomes stiffer upon binding of UL25 and gpD due to increased structural stability. At the same time the force required to break the capsid increases by ∼70% for both herpes and phage. This demonstrates a universal and evolutionarily conserved function of the minor capsid protein: facilitating the retention of the pressurized viral genome in the capsid. Since all eight human herpesviruses have UL25 orthologs, this discovery offers new opportunities to interfere with herpes replication by disrupting the precise force balance between the encapsidated DNA and the capsid proteins crucial for viral replication.

Highlights

Herpesviruses consist of a double-stranded-DNA-filled capsid, an unstructured protein layer and a lipid envelope
We show that binding of UL25 in herpes simplex virus type 1 (HSV-1) provides similar capsid reinforcement as gpD binding in phage ␭: the breaking force for both herpes and phage capsids is increased by ∼70% [obtained from force at which the capsid breaks (Fbreak) (C-capsid) / Fbreak (UL25-null A-capsid) and Fbreak / Fbreak]
We demonstrate that it is only the UL25 protein that reinforces the HSV-1 capsid, not the whole capsid vertex-specific component’ (CVSC) complex around the pentons; the UL17 portion of CVSC does not contribute to the reinforcement effect

Summary

Introduction

Herpesviruses consist of a double-stranded (ds)-DNA-filled capsid, an unstructured protein layer (the tegument) and a lipid envelope. Analogous to dsDNA bacteriophages, herpesviruses package their micrometer-long genome into a nanometerscale capsid. The stressed state of packaged viral DNA generates an internal pressure of tens of atmospheres [4,5] This pressure is capable of powering genome ejection from the capsid and has been characterized for several bacterial viruses [4,6,7] and an archeal virus, His1 [8]. Alterations of this internal pressure influence viral infectivity [9] and replication [10]. Evolutionary conservation of internal pressure in viruses that infect hosts from each of the three domains of life suggests it is a key mechanism for viral infection

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