Classification and Interactions of LRR Receptors and Co-receptors Within the Arabidopsis Plasma Membrane - An Overview

Abstract

Receptor kinases (RK) constitute the largest protein kinase family in plants. In particular, members of the leucine-rich repeat-receptor kinases (LRR-RKs) are involved in the perception of various signals at the plasma membrane. Experimental evidence over the past years revealed a conserved activation mechanism through ligand-inducible heterodimer formation: a ligand is recognized by a receptor kinase with a large extracellular domain (ECD). This ligand binding receptor directly interacts with a so-called co-receptor with a small ECD for ligand fixation and kinase activation. A large proportion of LRR-RKs is functionally still uncharacterized and the dynamic complexity of the plasma membrane makes it difficult to precisely define receptor kinase heterodimer pairs and their functions. In this review, we give an overview of the current knowledge of LRR receptor and co-receptor functions. We use ECD lengths to classify the LRR receptor kinase family and describe different interaction properties of ligand-binding receptors and their respective co-receptor from a network perspective.

Keywords: LRR-RKs; co-receptor; extracellular domain lengths; ligand-perceiving; plasma membrane.

FIGURE 1

Classification of LRR-RKs by extracellular domain lengths. (A) Histogram of ECD lengths distribution. Light blue bars represent proteins with ECD shorter than 400 amino acids, in the following named “co-receptor group.” Light red bars represent proteins with ECD longer than 400 amino acids, in the following named “ligand-perceiving receptor group.” (B) Pie charts of subfamily composition in the “co-receptor group.” (C) Pie charts of subfamily composition in the “ligand-perceiving receptor group.” The square boxes contain gene symbols of LRR-RKs with known functions. Red font color: SERKs and their correspondence ligand binding receptors; Green font color: CIKs and their correspondence ligand binding receptors; Purple font color: MDIS1 and its interacting receptors MIKs.

FIGURE 2

Interactions between ligand-perceiving receptors and co-receptors from published databases. (A) Network demonstration of 270 interaction events. Two interaction modes are marked with colored edges. Cyan edges: Mode-I, “co-receptor” is the center node surrounded by “ligand-perceiving receptors”; red edges: Mode-II, “ligand-perceiving receptor” is the center node surrounded by “co-receptors.” Blue ellipse: LRR-RKs from “co-receptor group”; Pink diamond: LRR-RKs from “ligand-perceiving group”. Width of edges: interaction values from STRING, MIND, TAIR and AI databases. (B) SERK3/BAK1 (AT4G33430) centered sub-network. (C) BAM1 (AT5G65700) centered sub-network.