Plant Cyclic Nucleotide-Gated Channels: New Insights on Their Functions and Regulation

Abstract

Recent advances of plant cyclic nucleotide-gated channels give new insight into their molecular functions focusing on regulation, subunit assembly, and phosphorylation.

Figure 1.

Model of a CNGC-containing signal complex (nanodomain/channelosome, shown in darker gray). A heterotetrameric CNGC channel is part of a sensing receptor complex containing PRRs, their clients (e.g. BIK1, RESPIRATORY BURST OXIDASE HOMOLOG D, RLCK), various pumps (e.g. proton ATPase and Ca²⁺ pumps), and decoders (e.g. CPKs and CaM). The formation of such a signal complex can be permanent or temporal upon recognition of specific stimuli (transient signaling complex) and the combination of specific players can contribute to generate precise spatiotemporal Ca²⁺ signals. Recruitment of CNGCs in a specific signaling complex may be achieved by MLO proteins. Phosphorylation plays significant roles to activate CNGCs or induce their turnover by E3 ubiquitin ligases and the 26s proteasome. V, Vesicle.

Figure 2.

Three different modules of CaM regulation in CNGC complexes. A, When apo-CaM is bound to the IQ domain in the presence of low Ca²⁺-concentration, the homo-tetrameric CNGC12 channel or the CNGC8-CNGC18 hetero-complex can be gated open upon hyperpolarization to allow Ca²⁺ entry. B, Elevation of cytosolic Ca²⁺-concentration induces dissociation of Ca²⁺-CaM, which leads to closure of CNGC8-CNGC18. C, Alternatively, as shown for CNGC14 and CNGC2-CNGC4 hetero-complexes, binding of (Ca²⁺)-CaM induces conformational rearrangements resulting in channel closure. Red dots and red color indicate Ca²⁺ ions and high Ca²⁺ concentration, respectively. CaM is shown with its NT and CT lobes with two apo (white) or Ca²⁺ (red) loaded EF-hands. For clarity, two CT domains of CNGC subunits are shown for each complex only. The conserved helical parts of the cyclic nucleotide binding domain (CNBD) are represented by green rods, the helical IQ domain in purple. The open state (A) is symbolized by a Ca²⁺-occupied pore and a compact arrangement of the CNBD with the transmembrane part of the channel, in analogy to known structures (Li et al., 2017). In B and C, the pore is closed and the CNBD is separated from the membrane via the C-linker, to illustrate the closed state.