Rotaviruses – Characteristics

The morphological and biochemical characteristics of rotaviruses include the following:

a)      Mature virus particles are nonenveloped and possess a multilayered icosahedral protein capsid, of about 75nm in diameter, composed of an outer layer, an inner layer, and a core;
b)      The virus genome consists of 11 segments of double-stranded RNA;
c)      Viral particles contain an RNA-dependent RNA polymerase and other enzymes capable of producing capped RNA transcripts;
d)      Viral replication occurs in the cytoplasm of infected cells.
e)      The viruses are capable of genetic reassortment;
f)        Virus cultivation in vitro is facilitated by treatment with proteolytic enzymes, which enhances infectivity by cleavage of the outer capsid polypeptide VP4;
g)      Virus particles are formed by budding into the endoplasmic reticulum and
h)      Mature virion particles are liberated from infected cells by cell lysis.

Classification

Rotaviruses are included in the genus Rotavirus, in the family Rotaviridiae. They are classified serologically first into groups (or serogroups) containing viruses that share cross-reacting antigens detectable by serological tests such as immunofluorescence, ELISA and immunoelectro-microscopy. Six distinct groups (A to F) of rotaviruses have been described. Group A, B, and C rotaviruses have been found both in humans and animals; groups D, E, and F rotaviruses have been found only in animals. Group A rotaviruses have clearly been established as causing severe diarrhoeal disease in young; group B rotaviruses include those viruses associated with animal epidemics of severe diarrhoea primarily in adults in China. Group C viruses have been found sporadic cases and outbreaks of diarrhoea in piglets and children.
Viruses within a serogroup are classified further in to serotypes.

Serotypes are defined by
a)      Plaque reduction
b)      Fluorescent – focus reduction or
c)      Neutralization assays by using antisera to purified virus particles prepared in hyper immunized animals.

The assays measure the reactivity of antibody with the two outer capsid proteins (VP4 and VP7), which induce antibodies with neutralizing activity.

Structure

When examined by negative stain electron microscopy, intact virus particles resemble a wheel, with short spokes and a well-defined rim. The name rotavirus (from the Latin ‘rota’, meaning wheel) was suggested on the basis of this characteristic. Three types of particles are observed by EM; they are, double-shelled, single-shelled, and core. Double-shelled particles are 76.5nm in diameter, single–shelled particles are 70.5nm and cores are 50nm.
Rotavirus particles possess icosahedral symmetry. The distinctive feature of the virus structure is the presence of 132 large channels spanning both shells and linking the outer surface with the inner core. Sixty spikes, at least 4.5nm in length and each with a knob at its distal end have been shown to extend from the smooth surface of the outer shell.

Genome

The rotaviral genome of 11 segments of dsRNA is contained with in virus core capsid. The total genome contains about 18522 base pairs. The structural proteins present in core particles (VP1, VP2 and VP3) are obvious. Nonstructural proteins play a scaffolding role.

Genome analysis

Rotaviruses are the only mammalian agents known to contain 11 segments of dsRNA. The genome of group A viruses is composed of four high-molecular weight dsRNA segments (segments 1 to 4), five middle – sized segments (segments 5 to 9) including a distinct triplet of segments (segments 7 to 9) and two smaller segments (segments 10 and 11).
The rotavirus genes code for structural proteins found in virus particles and for nonstructural particles found in infected cells but not in mature particles. The consensus is that the protein products (VP1 to VP4, VP5, VP6, VP7, V8) of six of the genome segments are structural proteins found in the virus particles and that the outer five genome segments code for nonstructural proteins.

Evolution of the Rotaviral genome

Comparisons of the sequence of individual segments of different rotaviruses of Group 1 have shown that changes occur through genetic shift and drift, i.e. genome reassortment and sequence changes within segments.
Recent studies indicate that the antigenic composition of particles may be influenced by the interactions of specific combinations of the two outer capsid proteins, by trypsin treatment of particles or by oligosachharide addition at different sites on the outer capsid glycoprotein.

Structure and function of rotavirus proteins Core and inner capsid proteins

VP1: VP1 is encoded by genome segment1 that makes up the rotavirus core particles.
VP2: VP2 is encoded by genome segment Z; it is the most abundant structural protein found in core particles and is the third most abundant protein in double shelled particles.
VP2 is the only structural protein shown to possess nucleic acid (dsRNA, ssRNA and dsDNA) binding activity when evaluated by RNA overlay protein blot assay
VP3: VP3, encoded by genome segment 3 is a minor structural protein that may co- migrate with the outer capsid protein VP4 in many gel systems.
VP6: VP6 is encoded by genome segment 6 and is the major structural protein in virus particles located on the outer surface of single-shelled particles.

Outer capsid proteins

VP4: is the protein product of genome segment 4 and it is a nonglycosylated outer capsid protein and a haemagglutinin in many virus strains. In the presence of trypsin, VP4 is cleaved into VPs (molecular weight approximately 60000) and VP8 (molecular weight approximately 28000) and this cleavage results in enhancement of viral infectivity.
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(Dr. Assumpta Sharon worked on rotaviruses at the Indian Institute of Science, Bangalore, and is now a Science Editor with M/s Macmillan India Ltd., Residency Road, Bangalore - 560 001)