Regulatory mechanisms of β‐glucan synthases in bacteria, fungi, and plants

Abstract

The evidence accumulated to date indicates that 1,3‐β‐glucan synthase (EC 2.3.1.12) and 1,4‐β‐glucan synthase (EC 2.4.1.12) are regulated by different effectors. Further that the same synthase has different effectors, depending upon its presence in green plants, fungi, and bacteria. Synthases from plants require divalent cations and β‐linked glucosides whereas fungal enzymes require neither cations nor β‐glucosides, but most require nucleoside triphosphates for activation. Two endogenous effectors have been characterized and shown to produce activation in vitro. One is 3′,5′‐cyclic diguanylic acid that is the activator of cellulose synthase in bacteria. The other is a β‐linked glucosyl dioleoyl diglyceride from mung bean, capable of activating synthases that produce both β‐(1–3) and β‐(1–4) products. The results of product analysis of the β‐linked glucoside activated reaction suggest that the synthesis of (1–3) and (1–4) glucosyl linkages may share a common enzyme in plants. All synthases utilize uridine 5′‐diphosphoglucose (UDPG) and are associated with the plasma membrane. Efforts to solubilize the synthases from cellular fractions enriched in plasma membranes have been generally successful. The purification of the soluble enzymes, however, remains a major obstacle to the full understanding of their regulation.