Identification of Secret Agent as the O- GlcNAc Transferase That Participates in Plum Pox Virus Infection

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Abstract
Serine and threonine of many nuclear and cytoplasmic proteins are posttranslationally modified with O-linked N -acetylglucosamine ( O -GlcNAc). This modification is made by O-linked N -acetylglucosamine transferases (OGTs). Genetic and biochemical data have demonstrated the existence of two OGTs of Arabidopsis thaliana , SECRET AGENT (SEC) and SPINDLY (SPY), with at least partly overlapping functions, but there is little information on their target proteins. The N terminus of the capsid protein (CP) of Plum pox virus (PPV) isolated from Nicotiana clevelandii is O -GlcNAc modified. We show here that O- GlcNAc modification of PPV CP also takes place in other plant hosts, N. benthamiana and Arabidopsis . PPV was able to infect the Arabidopsis OGT mutants sec-1 , sec-2 , and spy-3 , but at early times of the infection, both rate of virus spread and accumulation were reduced in sec-1 and sec-2 relative to spy-3 and wild-type plants. By matrix-assisted laser desorption ionization-time of flight mass spectrometry, we determined that a 39-residue tryptic peptide from the N terminus of CP of PPV purified from the spy-3 mutant, but not sec-1 or sec-2 , was O -GlcNAc modified, suggesting that SEC but not SPY modifies the capsid. While our results indicate that O- GlcNAc modification of PPV CP by SEC is not essential for infection, they show that the modification has a role(s) in the process.

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