NACL STRESS ENHANCES PROTEOLYTIC TURNOVER OF THE TONOPLAST H+-ATPASE OF CITRUS-SINENSIS - APPEARANCE OF A 35 KDA FRAGMENT OF SUBUNIT-A STILL EXHIBITING ATP-HYDROLYSIS ACTIVITY

Salinity stress caused a decrease in the relative amount of the subunits of the hydrophilic head of the tonoplast H+-ATPase from leaf cells of Valencia orange [Citrus sinensis (L.) Osbeck], In parallel, a 35 kDa polypeptide appeared in the tonoplast, which could be separated from the tonoplast H+-ATPase and cross-reacted with an antiserum against the catalytic subunit A of the tonoplast H+-ATPase. This polypeptide seems to show ATP hydrolysis activity and is considered to be a product of proteolytic breakdown of subunit A, This indicates an increased turnover of the tonoplast H+-ATPase of Valencia orange under salinity stress.