Cotton Leaf Curl Multan virus C4 protein suppresses both transcriptional and post-transcriptional gene silencing by interacting with SAM synthetase
Fig 7
Model for methyl cycle inactivation by geminiviral proteins.
In this figure, Met for L-methionine; SMA for S-adenosyl methionine; SAH for S-adenosyl homocysteine; Hcy for homocysteine; THF for tetrahydrofolate; PPi for pyrophosphate; Pi for inorganic phosphate. SAMS produce SAM from methionine. SAM is the methyl donor for most transmethylation reactions. The product, SAH, inhibits transmethylation by competing with SAM for methyltransferases (MTases). SAH is converted to homocysteine (Hcy) and adenosine by S-adenosyl homocysteine hydrolase (SAHH). Phosphorylation of adenosine by adenosine kinase (ADK) is critical because the SAHH-catalyzed reaction is reversible and the equilibrium lies in the direction of SAH synthesis. Geminivirus AC2/AL2 and C2/L2 proteins inactivate ADK and has also been shown to stabilize SAM decarboxylase (SAMDC), which causes decarboxylated SAM (dcSAM) levels to rise. TYLCCNV betasatellite encoded βC1 protein directly antagonizes the methyl cycle by inhibiting SAHH. Data in this report indicates that the CLCuMuV C4 protein inhibits SAMS activity by interacting directly.