Controls the tethering of the proton ATPase AHA1 to the plasma membrane. Is essential for stomatal opening in response to low concentration of carbon dioxide and light. Translocates reversibly between the cytoplasm and the plasma membrane depending on environmental conditions. Expressed in roots, cotyledons, leaves, stems and flowers. Expressed in guard cells and mesophyll cells of leaves. Retarded growth due to impaired stomatal movement. Belongs to the unc-13 family.