Sj. Craftsbrandner et al., THE 2 FORMS OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE ACTIVASE DIFFER IN SENSITIVITY TO ELEVATED-TEMPERATURE/, Plant physiology, 114(2), 1997, pp. 439-444
Ribulose-1,5-bisphosphate carboxylase/oxygenase activase often consist
s of two polypeptides that arise from alternative splicing of pre-mRNA
. In this study recombinant versions of the spinach (Spinacea oleracea
L.) 45- and 41-kD forms of activase were analyzed for their response
to temperature. The temperature optimum for ATP hydrolysis by the 45-k
D form was 45 degrees C, approximately 13 degrees C higher than the 41
-kD form. When the two forms were mixed, the temperature response of t
he hybrid enzyme was similar to the 45-kD form. In the absence of aden
ine nucleotide, preincubation of either activase form at temperatures
above 25 degrees C inactivated ATPase activity. Adenosine 5'-(gamma-th
io)triphosphate, but not ADP, significantly enhanced the thermostabili
ty of the 45-kD form but was much less effective for the 41-kD form. I
ntrinsic fluorescence showed that the adenosine 5'-(gamma-thio)triphos
phate-induced subunit aggregation was lost at a much lower temperature
for the 41-kD than for the 45-kD form. However, the two activase form
s were equally susceptible to limited proteolysis after heat treatment
. The results indicate that (a) the 45-kD form is more thermostable th
an, and confers increased thermal stability to, the 41-kD form, and (b
) a loss of subunit interactions, rather than enzyme denaturation, app
ears to be the initial cause of temperature inactivation of activase.