Evidence for a protein kinase cascade in higher plants: 3‐Hydroxy‐3‐methylglutaryl‐CoA reductase kinase

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 kinase, 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 carboxylase. The kinase is located in the soluble, 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 protein kinase cascade which has been highly conserved during evolution, since the kinase is inactivated and reactivated by mammalian protein phosphatases (2A or 2C) and mammalian kinase kinase, respectively. This contrasts with the situation for many other mammalian protein kinases 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.