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

Abstract

Summary 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 in sucrose gradients, which is less than the virion sedimentation rate of 180S to 200S. A protein of apparent M r 60000, 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 M r 60000 protein did not sediment in sucrose gradients in a compact zone as would be expected for a particle of uniform size. The M r 60000 protein was present at a concentration equal to or slightly higher (up to 400 µg/g leaf tissue) than the unencapsidated virus RNA (up to 300 µg/g leaf tissue). Sedimentation results support the conclusion that the virus RNA and the M r 60000 protein were combined in polydispersed nucleoprotein particles which may or may not have been attached to ribosomes or ribosome subunits. The M r 60000 protein was not serologically related to the capsid protein. Gold-labelled antibodies to the M r 60000 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.