Purification and Characterization of a Membrane-Bound Protease from Chlamydomonas reinhardtii

In Chlamydomonas reinhardtii y-1, newly synthesized chlorophyll a/b-binding apoproteins are degraded when chlorophylls are not present for assembly of stable light-harvesting complexes. A protease was purified from the membrane fraction of degreened y-1 cells, which digested chlorophyll a/b-binding proteins in membranes from C. reinhardtii pg-113, a protease-deficient strain. This protease was active with p-nitroanilides of nonpolar amino acids (Leu and Phe), but not of basic amino acids (Lys and Arg). The apparent molecular weight of the enzyme is 38,000 +/- 2,000 as determined by electrophoresis in the presence of sodium dodecyl sulfate. Typical inhibitors of the major classes of proteases were ineffective with this enzyme. Protease activity was constant from pH 7.5 to 9; a plot of log V versus pH suggested that deprotonation of an ionizable group with a pK value of 6.0 to 6.5 is required for activity. The protease was inactivated by diethylpyrocarbonate and by photooxidation sensitized by rose bengal. These results suggested that a histidyl residue is required for catalysis. Although very sensitive to photodynamic conditions in vitro, the enzyme was not inactivated in vivo when cells were exposed to light.