Identification and characterization of SppA, a novel light-inducible chloroplast protease complex associated with thylakoid membranes

A new component of the chloroplast proteolytic machinery from Arabidopsis t haliana was identified as a SppA-type protease. The sequence of the mature protein, deduced from a full-length cDNA, displays 22% identity to the seri ne-type protease IV (SppA) from Escherichia coli and 27% identity to Synech ocystis SppA1 (sll1703) but lacks the putative transmembrane spanning segme nts predicted from the E. coli sequence. The N-terminal sequence exhibits t ypical features of a cleavable chloroplast stroma-targeting sequence. The c hloroplast localization of SppA was confirmed by in organello import experi ments using an in vitro expression system and by immunodetection with antig en-specific antisera. Subfractionation of intact chloroplasts demonstrated that SppA is associated exclusively with thylakoid membranes, predominantly stroma lamellae, and is a part of some high molecular mass complex of abou t 270 kDa that exhibits proteolytic activity. Treatments with chaotropic sa lts and proteases showed that SppA is largely exposed to the stroma but tha t it behaves as an intrinsic membrane protein that may have an unusual mono topic arrangement in the thylakoids. We demonstrate that SppA is a light-in ducible protease and discuss its possible involvement in the light-dependen t degradation of antenna and photosystem Il complexes that both involve ser ine-type proteases.