RUBISCO ACTIVASE, A POSSIBLE NEW MEMBER OF THE MOLECULAR CHAPERONE FAMILY

The present research addresses the question of whether Rubisco activas e (R-A), the enzyme reported to activate Rubisco, is actually a molecu lar chaperone rather than a conventional enzyme, Several biochemical p roperties known to be characteristics of molecular chaperones were tes ted for R-A with positive results. The experiments were performed eith er in vitro with purified spinach Rubisco and Rubisco activase or in v ivo in maize seedling leaves. Our results confirmed that activation of Rubisco by R-A is an ATP hydrolysis-dependent process and further dem onstrated that (a) R-A binds preferably to nonnative Rubisco protein, than to the native form, and dissociates from this complex after addit ion of ATP, (b) R-A increases during heat shock treatment in maize see dling leaves, and (c) a large recovery of Rubisco activity is achieved from heat-inactivated Rubisco by addition of R-A and an energy source . We conclude that R-A characteristics strongly suggest that this prot ein belongs to the molecular chaperone group. The possible role of R-A on maintaining Rubisco activity in vivo is discussed.