Pyrabactin resistance 1 (PYR1), PYR1-like (PYL), regulatory component of abscisic acid receptors (RCARs), and related proteins
The PYR/PYL/RCAR-like family belongs to the SRPBCC (START/RHO_alpha_C/PITP/Bet_v1/CoxG/CalC) domain superfamily of proteins that bind hydrophobic ligands. SRPBCC domains have a deep hydrophobic ligand-binding pocket. PYR/PYL/RCAR plant proteins are receptors involved in signal transduction. They bind abscisic acid (ABA) and mediate its signaling. ABA is a vital plant hormone, which regulates plant growth, development, and response to environmental stresses. Upon binding ABA, these plant proteins interact with a type 2C protein phosphatase (PP2C), such as ABI1 and ABI2, and inhibit their activity. When ABA is bound, a loop (designated the gate/CL2 loop) closes over the ligand binding pocket, resulting in the weakening of the inactive PYL dimer and facilitating type 2C protein phosphatase binding. In the ABA:PYL1:ABI1 complex, the gate blocks substrate access to the phosphatase active site. A conserved Trp from PP2C inserts into PYL to lock the receptor in a closed formation. This group also contains Methylobacterium extorquens AM1 MxaD. The mxaD gene is located within the mxaFJGIR(S)ACKLDEHB cluster which encodes proteins involved in methanol oxidation. MxaD may participate in the periplasmic electron transport chain for oxidation of methanol. Mutants lacking MxaD exhibit a reduced growth on methanol, and a lower rate of respiration with methanol.