A Non-capsid Protein Associated with Unencapsidated Virus RNA in Barley Infected with Barley Stripe Mosaic Virus

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Abstract
Barley tissue with an acute systemic infection of barley stripe mosaic virus contained a large amount of unencapsidated virus RNA which was stable in extracts made in ribosome isolation buffer. The virus RNA in ribosome preparations sedimented in a broad band at 80S to 100S sucrose gradients, which is less than the virion sedimentation rate of 180S to 200S. A protein of apparent Mr 60,000, which sedimented with the virus RNA, was present in ribosome extracts from infected plants but absent from those from uninfected plants. The protein is probably a virus protein because its apparent molecular weight varied slightly with the strain of virus. The structure containing the Mr 60,000 protein did not sediment in sucrose gradients in a compact zone as would be expected for a particle of uniform size. The Mr 60,000 protein was present at a concentration equal to or slightly higher (up to 400 .mu.g/g leaf tissue) than the unencapsidated virus RNA (up to 300 .mu.g/g leaf tissue). Sedimentation results support the conclusion that the virus RNA and the Mr 60,000 protein were combined in polydispersed nucleoprotein particles which may or may not have been attached to ribosomes or ribosome subunits. The Mr 60,000 protein was not serologically related to the capsid protein. Gold-labelled antibodies to the Mr 60,000 protein stained the cytoplasm in thin sections of infected cells but not that of uninfected cells. However, no distinct immunogold-labelled particle could be identified as the presumed nucleoprotein.