Avian Reovirus Polypeptides: Analysis of Intracellular Virus-Specified Products, Virions, Top Component, and Cores

Abstract

Avian reovirus-specified polypeptides can be separated into three size classes: large (λ), medium (μ), and small (σ), similar to those of the mammalian reoviruses. A nomenclature has been proposed to indicate the individual polypeptides within each size class by progressive alphabetical subscripts. Three λ polypeptides (λ A , λ B , and λ C ) are found in infectious viral particles and have molecular weights of 145,000, 130,000, and 115,000, respectively. All are present in core preparations, and two (λ A and λ B ) appear to be exposed at the surface of the virion. Two μ polypeptides can be distinguished in purified virus (μ A , 72,000 daltons; μ B , 70,000 daltons), and another is occasionally evident by immunoprecipitation from infected-cell extracts (μ NS ). μ B represents the major outer capsid protein and is structurally homologous to μ 1C of the mammalian reoviruses. No additional μ proteins can be detected, and there is no evidence for a product-precursor relationship among these proteins. Three major σ proteins are evident in viral particles. σ C has the lowest molecular weight, is part of the outer capsid of the virion, and appears homologous to the mammalian σ 1 protein. Interestingly, it demonstrates the greatest polymorphism of all the polypeptides among the different avian reoviruses examined. σ B (36,000 daltons) is a major constituent of the outer capsid and, like σ C , is exposed to the surface of the virion. σ A (39,000 daltons) appears to be an internal protein. An additional polypeptide band in the σ class having an apparent molecular weight of 34,000 to 35,000 can be seen under three different conditions: (i) in some S1133 reovirus preparations, particularly after prolonged storage, a new band (σ B′ ) appears with a reduction in intensity of σ B , suggesting that σ B′ is a degradation product of σ B ; (ii) in polypeptides immunoprecipitated from infected-cell extracts, a major band (σ NS ) is apparent migrating just ahead of σ B ; (iii) in top component preparations from all avian reoviruses examined, a band (σ TC ) with mobility identical to that of σ NS represents a major constitutent and appears to be incorporated within the particle itself. The relationship among these three bands is not currently known. Avian reovirus polypeptides are thus in general similar to those found in mammalian reoviruses, but differences do exist which may be important for understanding viral structure and assembly.