Rotavirus non-structural proteins: structure and function

The replication of rotavirus is a complex process that is orchestrated by an exquisite interplay between the rotavirus non-structural and structural proteins. Subsequent to particle entry and genome transcription, the non-structural proteins coordinate and regulate viral mRNA translation and the formation of electron-dense viroplasms that serve as exclusive compartments for genome replication, genome encapsidation and capsid assembly. In addition, non-structural proteins are involved in antagonizing the antiviral host response and in subverting important cellular processes to enable successful virus replication. Although far from complete, new structural studies, together with functional studies, provide substantial insight into how the non-structural proteins coordinate rotavirus replication. This brief review highlights our current knowledge of the structure-function relationships of the rotavirus non-structural proteins, as well as fascinating questions that remain to be understood.

Introduction

The rotavirus genome consists of 11 double-stranded (ds)RNA segments, and each segment encodes one protein with the exception of segment 11, which in some rotavirus strains codes for 2 proteins, NSP5 and NSP6 [1, 2]. Of these proteins encoded by the viral genome, six are structural (VPs) and the remaining are non-structural (NSPs). The complex architecture of the mature rotavirus particle, with three concentric capsid layers, elegantly integrates the necessary elements required for cell entry and endogenous transcription of viral mRNAs [3, 4]. Removal of the outer layer during the process of cell entry triggers the endogenous transcriptase activity of the resulting double-layered particles [1]. The capped transcripts are released through aqueous channels at the 5-fold axes of these intact particles [5]. Once these initial transcripts are translated, the rotavirus NSPs then coordinate various stages of genome replication and viral assembly by adapting and modifying the cellular machinery, which leads to productive release of mature particles through cell lysis. A brief overview of the rotavirus replication cycle is provided in Figure 1.

Although the functional roles served by NSP2, NSP3, NSP4, and NSP5 during rotavirus replication are relatively well characterized and are discussed below, the roles of NSP1 and NSP6 remain less clear. Owing to the lack of structural studies on NSP1 and NSP6, only a brief description of their function is included in this review. Early studies indicated that NSP1, which exhibits significant sequence variation among rotavirus strains, is not essential for rotavirus replication in cultured cells; however, more recent studies implicate NSP1 in host-range restriction, and in countering the innate host antiviral response and in suppressing induction of apoptosis during early stages of infection to promote viral growth [6, 7, 8, 9] (see article by Angel, Franco and Greenberg in this issue). Unlike the other rotavirus NSPs, NSP6 is not encoded by all rotavirus strains. For those strains that do express NSP6, it is translated from an open reading frame out-of-phase with that of NSP5 in segment 11 [2]. This 12-kDa protein has a high turnover rate and is degraded within 2 hours of synthesis [10]. NSP6 has shown sequence independent nucleic acid binding, with similar affinities for ssRNA and dsRNA [10]. Although some studies have suggested that this protein localizes to the viroplasm [10], the precise role of NSP6 in rotavirus replication remains to be characterized.