MECHANISM OF THE DECLINE IN VACUOLAR H+-ATPASE ACTIVITY IN MUNG BEAN HYPOCOTYLS DURING CHILLING

The mechanism responsible for the decrease in the activity of vacuolar H+-ATPase during chilling was investigated in seedlings of mung bean (Vigna radiata). After chilling at 0-degrees-C for 3 d, the activity of vacuolar H+-ATPase, calculated on the basis of membrane protein, decreased to 47% of the original value. Of the nine subunits of the ATPase, the specific contents of at least six subunits, of 68, 57, 44, 38, 37, and 32 kD, decreased in vacuolar membranes after chilling, as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These subunits were released by treatment with chaotropic anions such as thiocyanate. The level of the 16-kD subunit did not change. Immunoblot analyses showed the decrease in the levels of the subunits of 68, 57, and 32 kD. Furthermore, the specific activity of the ATPase purified from chilled hypocotyls was two-thirds of that of the enzyme from nonchilled seedlings, and the enzyme from chilled tissue retained only a small a mount of the 32-kD subunit. These results suggest that a selective release of the peripheral subunits of the ATPase from the membrane and a partial degradation of the ATPase complex may occur in vivo during chilling.