14-3-3s regulate global cleavage of their diverse binding partners in sugar-starved Arabidopsis cells

Abstract

Despite 14‐3‐3 proteins being implicated in the control of the eukaryotic cell cycle, metabolism, cell signalling and survival, little is known about the global regulation or functions of the phosphorylation‐dependent binding of 14‐3‐3s to diverse target proteins. We identified Arabidopsis cytosolic proteins that bound 14‐3‐3s in competition with a 14‐3‐3‐binding phosphopeptide, including nitrate reductase, glyceraldehyde‐ 3‐phosphate dehydrogenase, a calcium‐dependent protein kinase, sucrose‐phosphate synthase (SPS) and glutamyl‐tRNA synthetase. Remarkably, in cells starved of sugars or fed with non‐metabolizable glucose analogues, all 14‐3‐3 binding was lost and the target proteins were selectively cleaved into proteolytic fragments. 14‐3‐3 binding reappeared after several hours of re‐feeding with sugars. Starvation‐induced degradation was blocked by 5‐amino imidazole‐4‐carboxamide riboside (which is converted to an AMP‐mimetic) or the protease inhibitor MG132 (Cbz‐leu‐leu‐leucinal). Extracts of sugar‐starved (but not sugar‐fed) Arabidopsis cells contained an ATP‐independent, MG132‐sensitive, neutral protease that cleaved Arabidopsis SPS, and the mammalian 14‐3‐3‐regulated transcription factor, FKHR. Cleavage of SPS and phosphorylated FKHR in vitro was blocked by binding to 14‐3‐3s. The finding that 14‐3‐3s participate in a nutrient‐sensing pathway controlling cleavage of many targets may underlie the effects of these proteins on plant development.