Abstract
During Agrobacterium-mediated genetic transformation of plants, several bacterial virulence (Vir) proteins are translocated into the host cell to facilitate infection. One of the most important of such translocated factors is VirF, an F-box protein produced by octopine strains of Agrobacterium, which presumably facilitates proteasomal uncoating of the invading T-DNA from its associated proteins. The presence of VirF also is thought to be involved in differences in host specificity between octopine and nopaline strains of Agrobacterium, with the current dogma being that no functional VirF is encoded by nopaline strains. Here, we show that a protein with homology to octopine VirF is encoded by the Ti plasmid of the nopaline C58 strain of Agrobacterium. This protein, C58VirF, possesses the hallmarks of functional F-box proteins: it contains an active F-box domain and specifically interacts, via its F-box domain, with SKP1-like (ASK) protein components of the plant ubiquitin/proteasome system. Thus, our data suggest that nopaline strains of Agrobacterium have evolved to encode a functional F-box protein VirF.
Highlights
Members of the Agrobacterium genus are phytopathogenic bacteria with the unique ability to transfer and integrate a segment of their own DNA (T-DNA) into the genome of their plant hosts[1,2]
Whereas the ProfileScan software did not detect any functional domains in the C58VirF sequence, manual analysis of sequence alignment revealed a region of homology, corresponding to the octopine-type F-box domain, including some of the most conserved amino acid residues of the F-box domains[15,16]
A strong homology is found in the C-terminus of the protein, which corresponds to the arginine-rich bacterium-to-host cell translocation signal; this signal allows a Vir protein to be recognized as substrate by the bacterial type IV secretion system (T4SS), which transports it into the host cytoplasm[17]
Summary
Members of the Agrobacterium genus are phytopathogenic bacteria with the unique ability to transfer and integrate a segment of their own DNA (T-DNA) into the genome of their plant hosts[1,2]. The transfer and integration of Agrobacterium T-DNA relies on a set of proteins encoded by bacterial vir (virulence) genes located on a specialized Ti (tumor inducing)-plasmid, and on interaction of these Vir proteins with host cell factors[1,2]. VirF, one of these non-essential genes, was originally described as responsible for difference of virulence between two main A. tumefaciens strains: octopine and nopaline (named after the nature of the opines produced in tumors) on several specific hosts. Identified in prokaryotes[11] represents a bacterial pathogen effector that interferes with the host ubiquitin/ proteasome system (UPS)[13] That such an important virulence function as an F-box protein is not conserved between major Agrobacterium strains does not make biological sense. The level of virulence of octopine and nopaline strains of Agrobacterium on different hosts depends, at least in part, on specificity of their VirF F-box proteins
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