DEGRADATION OF THE LIGHT-STRESS PROTEIN IS MEDIATED BY AN ATP-INDEPENDENT, SERINE-TYPE PROTEASE UNDER LOW-LIGHT CONDITIONS

Green plants respond to light stress by induction of the light-stress proteins (ELIPs). These proteins are stable as long as the light stres s persists but are very rapidly degraded during subsequent low light c onditions [Adamska, I., Kloppstech, K. & Ohad, I. (1993) J. Biol. Chem . 268, 5438-5444]. Here we report that the degradation of ELIPs is med iated by an extrinsic, thylakoid-associated protease which is already present in the membranes during light stress conditions. Partial purif ication of the protease by perfusion chromatography indicates that thi s proteolytic activity may be represented by a protein with an apparen t molecular mass of 65 kDa. The ELIP-directed protease is localized in the stroma lamellae of the thylakoid membranes and does not require A TP or additional stromal factors for proteolysis. The protease has an optimum activity at pH 7.5-9.5 and requires Mg2+ for its activity. The ELIP-degrading protease show an unusual temperature sensitivity and b ecomes reversibly inactivated at temperatures below 20 degrees C and a bove 30 degrees C. Studies with protease inhibitors indicate that this enzyme belongs to the serine class of proteases. The enhanced degrada tion of ELIP in isolated thylakoid membranes after addition of the ion ophore nigericin suggests that a trans-thylakoid Delta pH or changes i n ionic strength may be involved in the mechanism of protease activati on.