Purification, properties, and subcellular localization of foxtail mosaic potexvirus 26-kDa protein.

The open reading frame 2 (ORF2) of the potexviral genome encodes a 24- to 26-kDa protein which is part of the "triple gene block," a group of overlapping ORFs also present in the genomes of the carla-, hordei-, and furoviruses. The product of these ORFs is believed to play a role in the cell-to-cell movement of the viruses in host plants. The amino acid sequences of the homologous ORF2 products encoded by these related viruses suggest that they specify NTP binding and possibly helicase activities. We have used an Escherichia coli expression system to produce significant amounts of the 26-kDa protein (p26) encoded by foxtail mosaic potexvirus ORF2. p28 was purified to near homogeneity by conventional purification methods and some of its biochemical properties were determined. We present evidence that p26 is an ATP, CTP, and RNA binding protein with apparent ATPase activity. Western blot analysis of infected plant extracts using a polyclonal antiserum produced against p26 indicates that it is a relatively stable protein maintained at high levels for at least 6 days following its peak level of expression. Moreover, it is found predominantly in the soluble fraction of infected tissues. An immunocytochemical analysis of infected Chenopodium quinoa leaves reveals that p26 is exclusively associated with cytoplasmic inclusions in proximity to but distinct from aggregates of viral particles.

Defective RNAs of clover yellow mosaic virus encode nonstructural/coat protein fusion products.

A small group of 1.2-kb RNAs present on polyribosomes from clover yellow mosaic virus (CYMV)-infected tissue contains sequences from the genomic RNA (gRNA) of CYMV and is encapsidated by CYMV coat protein. Some features of these RNAs suggest that they are similar to defective interfering (DI) RNAs, and would be the first to be reported for the potexvirus group. The prototype 1.2-kb RNA is 1172 nucleotides in length excluding a probable poly(A) tail and is composed of two noncontiguous regions corresponding to 757 nucleotides of the 5' and 415 nucleotides of the 3' termini of CYMV's gRNA. The sequence of the prototype 1.2-kb RNA reveals that the two terminal gRNA regions present in this RNA encode a single open reading frame (ORF) joining the N-terminus of the 191-kDa nonstructural product and the C-terminus of the coat protein to form a 35-kDa 191-kDa/coat protein fusion product. The coding properties of this prototype RNA have been confirmed by translation in vitro of native and synthetic transcripts of the 1.2-kb RNAs, both of which direct the synthesis of the anticipated 35-kDa product which reacts with anti-CYMV antiserum. Three additional 1.2-kb RNA species, each of which contains a unique junction site, have been characterized. In all cases, a fusion ORF encoding a 191-kDa/coat protein fusion product is encoded on the RNA. The presence of a fusion ORF in all members of the 1.2-kb RNA species analyzed suggests that maintenance of this ORF may be important for the survival of this class of RNA within the plant. This coding strategy represents a novel property of plant virus defective RNAs.

The structure of tubes of bovine rotavirus nucleocapsid protein (VP6) assembled in vitro.

The structure of tubes of reassembled nucleocapsid protein (VP6) from bovine rotavirus (BRV) was determined using optical diffraction of electron micrographs. The tubes consist of a five-start helix of hexagons, with 38 hexagons per helix in a true repeat of three turns. The morphological subunits comprising the hexagons are probably elongated trimers. The structure of naturally occurring tubes (D. Chasey and J. Labram, 1983, J. Gen. Virol. 64, 863-872) was also examined and shown to be similar but not identical to that of tubes assembled in vitro. Considerations of the assembly process are discussed.

The generality of cation-binding sites in rod-shaped viruses.

Hydrogen-ion titration curves have been measured for two filamentous plant viruses (clover yellow mosaic virus and potato virus X) and two filamentous bacterial viruses (fd and Pf1) with and without Ca2+ or Mg2+ ions present, and for the protein of the PM6 mutant of tobacco mosaic virus. The bacterial viruses do not possess the 'strong' cation-binding sites found in all plant viruses, but they have 'weak' sites that can be assigned to juxtaposed carboxylate groups on their external surfaces. The strong sites in plant viruses still cannot be assigned to any particular amino-acid side chains, but they must be located in the region of high electronegativity near the axis.

Melting of viral RNA by coat protein: assembly strategies for elongated plant viruses.

The coat proteins of two rod-shaped plant viruses, papaya mosaic virus (PMV) and tobacco mosaic virus (TMV), have been tested for RNA melting activity. Under reconstitution conditions, PMV protein melts RNA in a noncooperative fashion. This activity is aspecific and is inhibited by low concentrations of NaCl as is virus reconstitution. TMV protein does not melt RNA either in the absence of NaCl or under reconstitution conditions at moderate ionic strength levels. The results suggest that elongated plant viruses have evolved at least two different assembly strategies in order to satisfy the requirement that the RNA within these viruses be in a melted configuration.

The polarity of assembly of papaya mosaic virus and tobacco mosaic virus RNAs with PMV-protein under conditions of nonspecificity.

The problem of the rapid multiinitiation of papaya mosaic virus or tobacco mosaic virus RNA by PMV-protein near pH 7.0 at low ionic strength has been overcome. If NaCl is added to 0.1 M, both RNAs are first encapsidated at their respective 5' ends. This shows that the initial site of helix formation depends on the protein rather than the RNA.