In vivo detection, RNA-binding properties and characterization of the RNA-binding domain of the p7 putative movement protein from carnation mottle carmovirus (CarMV)

Biochemical and structural characterization studies on the p7 putative move ment protein from a Spanish isolate of carnation mottle carmovirus (CarMV) have been conducted. The CarMV p7 gene was fused to a sequence coding for a six-histidine tag and expressed in bacteria, allowing the purification of CarMV p7 and the production of a specific antiserum. This antiserum led to the immunological identification of CarMV p7 in infected leaf tissue from t he experimental host Chenopodium quinoa. Putative nucleic acid-binding prop erties of the CarMV p7 have been explored and demonstrated with both electr ophoretic mobility shift and RNA-protein blot in vitro assays using digoxig enin-labeled riboprobes. CarMV p7 did not show preferential binding to any of the different regions of the CarMV genomic RNA tested, suggesting that R NA binding was sequence nonspecific. Quantitative analyses of the data allo wed calculation of the apparent dissociation constant of the p7-RNA complex (K-d similar to 0.7 mu M) and supported a cooperative type of binding. A s mall 19-amino-acid synthetic peptide whose sequence corresponds to the puta tive RNA-binding domain of CarMV p7, at the basic central part of the prote in, was synthesized, and it was demonstrated that it binds viral RNA probes . Peptide RNA binding was as stable as p7 binding, although data indicated it was not cooperative, thus suggesting that this cooperative binding requi res another motif or motifs within the p7 amino acid sequence. The peptide could be induced to fold into an alpha-helix structure in which amino acids that are conserved among carmovirus p7-like proteins are distributed on on e side. This alpha-helix motif could define a new and previously uncharacte rized RNA-binding domain for plant virus movement proteins. (C) 1998 Academ ic Press.