Figure 1.

The Shaker K⁺ channel family. (A) Phylogenetic relationships in the grapevine (in green) and A. thaliana (in blue) Shaker K⁺ channel families. The Shaker family displays five groups in plants [115] named 1 to 5. Accession references are listed in Table 1. The abbreviations K_in, K_out K_weak, and K_silent are explained in the legend of Table 1. To find the conserved region, A. thaliana and grapevine Shaker polypeptide sequences were first aligned using MUSCLE 3.8.31 in full mode and then treated with Gblocks for alignment curation. The phylogenetic analyses were carried out using maximum likelihood with Phy ML 3.1/3.0 aLRT software. Tree rendering was performed using the tree drawing engine ETE 3 [116]). Bootstrap values are indicated at the corresponding nodes. The scale bar corresponds to a distance of 4,6 changes per 100 amino acid positions. (B) Functional Shaker channels are multimeric proteins formed by the assembly of four Shaker subunits. Current–voltage (I–V) curves illustrate the functional types found in the homotetrameric Shaker channels that form inwardly rectifying, weakly inwardly rectifying, or outwardly rectifying conductances. Int and ext: internal and external face of the plasma membrane. (C) Structural domains of a Shaker channel subunit. S1 to S6: transmembrane segments, CNBD: cyclic nucleotide-binding domain, ANKY: ankyrin domain (involved in protein-protein interactions, not found in all Shaker subunits), KHA: hydrophic and acidic domain. (D) Assembly of four Shaker alpha-subunit is a prerequisite for channel functioning. Three-dimensional representation of S1–S6 segments in a single subunit (left) or Shaker tetramers (right). Subunits are encoded either by the same gene (homotetrameric channel) or by different genes (heterotetrameric channel). K_in sub-units (Groups 1, 2, 3 and 4 in A) assemble as K_in channels, whereas K_out sub-units (Group 5) form K_out channels. No assembly could be detected between K_in and K_out channel subunits [117]. Stoichiometry studies have revealed the various possible combinations between the different subunits [117,118,119].