Thylakoid-bound proteolytic activity against LHC II apoprotein in bean

Abstract

Triton X-100 solubilized thylakoids, isolated from Phaseolus vulgaris chloroplasts, degrade endogenous or exogenously added LHC II. The degradation, as monitored by immunodetection of the remaining LHC II after incubation at 37°C, is activated by Mg⁺⁺ and inhibited by pCMB, EDTA, PMSF and benzamidine; the activity under high light conditions parallels chlorophyll photooxidation. The thylakoid-bound proteolytic activity is under phytochrome control. Etiolated plants pretreated by a white light pulse, and kept in the dark thereafter, show enhanced proteolytic activity, which follows rhythmical oscillations. On the other hand, chloramphenicol pretreatment of etiolated plants, prior to their transfer to continuous light, reduces the proteolytic activity against LHC II. The results suggest that the degradation involves a serine type protease, which depends on SH group(s), coded by the plastid genome; the protease action on LHC II is regulated by Mg⁺⁺, phytochrome, the biological clock and chlorophyll accumulation in the thylakoid. The stroma lamellar fraction, separated from French press disrupted chloroplasts, exhibits higher activity towards exogenous LHC II than the grana fraction. The stroma of intact chloroplasts exhibits also high proteolytic activity, which is drastically reduced when the lysis medium is supplemented with cations. This suggests that the protease is bound mainly on stroma lamellae and peripheral granal membranes, its association to the membranes being possibly under cation control.