| Research Abstract |
The cold stability of vacuolar H^+-ATPase in vivo is markedly different between mung bean (Vigna radiata L.) and pea (Pisum sativum L.), the plants which are representatives of cold-sensitive and cold-tolerant species, respectively. Upon cold exposure of mung bean seedlings the H^+-ATPase activity declined significantly soon after the start of the cold treatment. In pea seedlings, by contrast, the H^+-ATPase remained highly active for long periods of the cold treatment. On cold incubation of isolated vacuolar membranes in vitro in the presence of MgATP and chaotropic anions such as C1^-, NO^-_2 and NO^-_3, the vacuolar H^+-ATPases exhibited also a marked difference in the cold-induced destabilization between these plants, especially in the sensitivity to the concentration of the anions. The enzyme from mung bean appeared to be more susceptible to the anions, particularly to NO^-_2, than the enzyme from pea, resulting in more severe cold-inactivation at the lower concentrations. The MgATP-dependent cold inactivation of the H^+-ATPase was closely associated with a release of the peripheral sectors from the membranes in the same way as in vacuolar-type H^+-ATPases from wide range of organisms. Thus, the cold stability of the enzyme complex both in vivo and in vitro is considered to be dependent on the intrinsic properties of the enzyme, suggesting the existence of "cold-labile" and "cold-stable" types in plants. SDS-PAGE and immunoblot analysis of the subunit components of the H^+-ATPase from both plants revealed that the molecular structures, especially the epitopes of the membrane sectors (16kD), differ each other, albeit that the molecular masses of other minor subunits are also slightly different. These findings in the present study are considered to provide a new insight into the mechanism to control the cold stability of vacuolar H^+-ATPase in plants.
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