Evidence for a protein kinase cascade in higher plants

Abstract

Protein phosphorylation is well established as a regulatory mechanism in higher plants, but only a handful of plant enzymes are known to be regulated in this manner, and relatively few plant protein kinases have been characterized. AMP-activated protein kinase regulates key enzymes of mammalian fatty acid, sterol and isoprenoid metabolism, including 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase. We now show that there is an activity in higher plants which, by functional criteria, is a homologue of the AMP-activated protein kinasc, although it is not regulated by AMP. The plant kinase inactivates mammalian HMG-CoA reductase and acetyl-CoA carboxylase, and peptide mapping suggests that it phosphorylates the same sites on these proteins as the mammalian kinase. However, with the target enzymes purified from plant sources, it inactivates HMG-CoA reductase but not acetyl-CoA carboxylasc. The kinasc is located in the solublc, and not the chloroplast, fraction of leaf cells, consistent with the idea that it regulates HMG-CoA reductase, and hence isoprenoid biosynthesis, in vivo. The plant kinase also appears to be part of a protcin kinase cascade which has been highly conserved during evolution, since the kinase is inactivated and reactivated by mammalian protein phosphatases (2A or 2 C) and mammalian kinase kinase, respectively. This contrasts with thc situation for many othcr mammalian protein kinascs involved in signal transduction, which appear to have no close homologue in higher plants. 'To our knowledge, this represents the first direct evidence for a protein kinase cascade in higher plants.

Adenosine Monophosphate-Activated Protein Kinase: A Central Regulator of Metabolism with Roles in Diabetes, Cancer, and Viral Infection

“…AMPK orthologs are also found in plants (Alderson et al 1991;Mackintosh et al 1992). Deletion of the two catalytic subunit genes in a primitive plant, the moss Physcomitrella patens, produced no phenotype when the plants were grown in continuous light, but the plants failed to grow on more physiological alternate light/dark cycles (Thelander et al 2004).…”

Biochemical characterization of two forms of 3‐hydroxy‐3‐methylglutaryl‐CoA reductase kinase from cauliflower (Brassica oleracia)

“…The purification procedure used previously [15] to partially purify a single kinase activity from cauliflower, has been extensively modified. Extracts were prepared in a Waring blender by homogenizing 1.5 kg cauliflower inflorescence plus 25 g polyvinylpolypyrrolidone with 1.5 1 homogenization buffer (50 mM TrisEiCl, pH 8.2, 0.25 M mannitol, 50 mM NaF, 1 mM EDTA, 1 mM EGTA, 1 mM dithiothreitol, 1 mM benzamidine, 0.1 mM phenylmethane sulphonyl fluoride).…”

“…The one difference found to date is that the plant protein kinase is not activated by AMP. Since the plant lunase inactivates HMG-CoA reductase from potato tubers, and the phosphorylation site defined on mammalian HMG-CoA reductase is conserved in a number of plant HMG-CoA reductases, we have provisionally named the plant kinase HMG-CoA reductase kinase [15].In this study we report the further purification and characterization of the HMG-CoA reductase kinase protein from cauliflower inflorescences. During the course of this work we identified a second HMG-CoA reductase kinase activity which appears to represent a distinct gene product.…”

“…phosphoenolpyruvate carboxylase [8,9]. A number of higher plant protein kinases [lo-131 and protein phosphatases [14] have also been defined by cDNA cloning.Recently we obtained the first direct biochemical evidence for a protein kinase cascade in higher plants [15]. The protein kinase which is the central component of this cascade appears by functional criteria to be a homologue of the AMPactivated protein kinase, an activity which in mammalian cells is a component of the response to cellular stress [l] such as heat shock and hypoxia (unpublished results).…”

“…The one difference found to date is that the plant protein kinase is not activated by AMP. Since the plant lunase inactivates HMG-CoA reductase from potato tubers, and the phosphorylation site defined on mammalian HMG-CoA reductase is conserved in a number of plant HMG-CoA reductases, we have provisionally named the plant kinase HMG-CoA reductase kinase [15].…”