Degradation of the Light‐Stress Protein is Mediated by an ATP‐Independent, Serine‐Type Protease Under Low‐Light Conditions

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Abstract
Green plants respond to light stress by induction of the light‐stress proteins (ELIPs). These proteins are stable as long as the light stress persists but are very rapidly degraded during subsequent low light conditions [Adamska, I., Kloppstech, K. & Ohad, I. (1993) J. Biol. Chem. 268, 5438–5444]. Here we report that the degradation of ELIPs is mediated by an extrinsic, thylakoid‐associated protease which is already present in the membranes during light stress conditions. Partial purification of the protease by perfusion chromatography indicates that this proteolytic activity may be represented by a protein with an apparent molecular mass of 65 kDa. The ELIP‐directed protease is localized in the stroma lamellae of the thylakoid membranes and does not require ATP 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 becomes reversibly inactivated at temperatures below 20°C and above 30°C. Studies with protease inhibitors indicate that this enzyme belongs to the serine class of proteases. The enhanced degradation of ELIP in isolated thylakoid membranes after addition of the ionophore nigericin suggests that a trans‐thylakoid pH or changes in ionic strength may be involved in the mechanism of protease activation.