Multilevel Control of Arabidopsis 3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase by Protein Phosphatase 2A

Abstract

Plants synthesize a myriad of isoprenoid products that are required both for essential constitutive processes and for adaptive responses to the environment. The enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) catalyzes a key regulatory step of the mevalonate pathway for isoprenoid biosynthesis and is modulated by many endogenous and external stimuli. In spite of that, no protein factor interacting with and regulating plant HMGR in vivo has been described so far. Here, we report the identification of two B99 regulatory subunits of protein phosphatase 2A (PP2A), designated B99a and B99b, that interact with HMGR1S and HMGR1L, the major isoforms of Arabidopsis thaliana HMGR. B99a and B99b are Ca 2+ binding proteins of the EF-hand type. We show that HMGR transcript, protein, and activity levels are modulated by PP2A in Arabidopsis. When seedlings are transferred to salt-containing medium, B99a and PP2A mediate the decrease and subsequent increase of HMGR activity, which results from a steady rise of HMGR1-encoding transcript levels and an initial sharper reduction of HMGR protein level. In unchallenged plants, PP2A is a posttranslational negative regulator of HMGR activity with the participation of B99b. Our data indicate that PP2A exerts multilevel control on HMGR through the fivemember B99 protein family during normal development and in response to a variety of stress conditions.

Atypical Protein Phosphatase 2A Gene Families Do Not Expand via Paleopolyploidization

“…Two or more B56 isoforms regulate dephosphorylation of the BAK1 coreceptor (Segonzac et al, 2014), the brassinosteroid-signaling transcription factor BZR1 (Tang et al, 2011), and the brassinosteroid receptor BRI1 (Wang et al, 2016). Both Arabidopsis B55 isoforms interact with nitrate reductase (Heidari et al, 2011), and at least two B72 (clade II) isoforms interact with 3-hydroxy-3-methylglutaryl-CoA reductase (Leivar et al, 2011). These data suggest that, in plants at least, many PP2A subunit isoforms share some functional overlap.…”

“…Arabidopsis B subunits have been named according to the convention for mammalian isoforms: B, B9, or B99 followed by a Greek letter (Terol et al, 2002;Farkas et al, 2007;Leivar et al, 2011). As noted recently for the B56 family (Sommer et al, 2015), this convention is easily misconstrued to imply orthology with mammalian isoforms.…”

“…Our analyses show that FASS/TON2 is the ortholog of mammalian B99g, but under the current nomenclature, other isoforms are designated as Arabidopsis B99g (Table I). Because the shared use of Greek characters misleadingly suggests orthology between plant and animal B subunit isoforms, and because of the conflict between two published nomenclatures for Arabidopsis B99 subunits (Farkas et al, 2007;Leivar et al, 2011), we prefer a numerical system ( Table I) analogous to that used for Arabidopsis A and C subunits.…”

Proliferation and Morphogenesis of the Endoplasmic Reticulum Driven by the Membrane Domain of 3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase in Plant Cells

“…However, we found that OSER structures also accumulate endogenous HMGR protein, with concomitantly reduced HMGR transcript levels. Thus, the membrane availability induced by 1S:GFP somehow stabilizes the chimera and also, the endogenous HMGR, which was previously found in several plants to be submitted to ongoing degrading processes (Korth et al, 2000;Leivar et al, 2011;Pollier et al, 2013). Other endogenous and heterologous ER membrane proteins are also stabilized in OSER structures (Kochevar and Anderson, 1987;McLaughlin et al, 1999).…”

Suppressing Farnesyl Diphosphate Synthase Alters Chloroplast Development and Triggers Sterol-Dependent Induction of Jasmonate- and Fe-Related Responses

“…This is fully consistent with the hypothesis that variations of HMGR activity in response to changes in the flux of the MVAderived isoprenoid pathway occur mainly via posttranslational control , although the precise nature of this regulatory mechanism remains to be established. HMGR activity levels in FPSsilenced plants may be up-regulated through changes in the phosphorylation status of the enzyme (Dale et al, 1995;Douglas et al, 1997;Leivar et al, 2011) and/or protein degradation mechanisms involving the ERAD protein quality control system (Doblas et al, 2013;Pollier et al, 2013). In any case, an enhancement of HMGR activity concomitant to a reduction of FPS activity could lead to the accumulation of MVA pathway intermediates upstream of FPP that might cause the observed phenotype.…”