Modulation of Stress Granule Dynamics by Phosphorylation and Ubiquitination in Plants

The Arabidopsis tandem CCCH zinc finger 1 (TZF1) is an RNA-binding protein that plays a crucial role in plant growth and stress response. TZF1 can localize to ribonucleoprotein (RNP) granules in response to various abiotic stresses. However, very little is known about the composition, function, and assembly mechanism of plant RNP granules. In this report, we show that TZF1 contains two intrinsically disordered regions (IDRs) necessary for its localization to stress granules (SGs), a subclass of RNP granules. TZF1 recruits mitogen-activated protein kinase (MAPK) signaling components and an E3 ubiquitin ligase KEEP-ON-GOING (KEG) to SGs. TZF1 is phosphorylated by MPKs and ubiquitinated by KEG. The phosphorylation sites of TZF1 were mapped by mass spectrometry. Mutant studies revealed that phosphorylation and ubiquitination of specific residues played differential roles in enhancing or reducing TZF1 SG assembly and protein-protein interaction with mitogen-activated kinase kinase 5 (MKK5) in SGs. TZF1 is extremely unstable, and its accumulation can be enhanced by proteosome inhibitor MG132. We showed that TZF1 was ubiquitinated in vivo and in vitro by KEG and TZF1 accumulated at a much lower level in gain-of-function mutant keg-4, compared to the WT. Ubiquitination appeared to play a positive role in TZF1 SG assembly, because either single or higher order mutations caused reduced number of SGs per cell, while enhanced the coalescence of small SGs into a large nucleus-like SG encompassing the nucleus. Together, our results demonstrate that the assembly of TZF1 SGs is distinctively regulated by phosphorylation and ubiquitination.