Comparison of Properties of the Proteolytic Degradation of Unassembled Nuclear‐encoded Subunits of Ribulose‐1,5‐bisphosphate Carboxylase and of the Coupling Factor of Photophosphorylation CF₁*

The proteolytic degradation of unassembled small subunit polypeptides of ribulose‐1,5‐bisphosphate carboxylase and of the δ‐subunit of the coupling factor of photophosphorylation CF 1 were analyzed and compared in vitro in the presence of stroma or membrane preparations from ribosome‐deficient plastids isolated from 32°C‐grown rye leaves ( Secale cereale L.). Extracts obtained from 70S ribosome‐deficient rye leaves after radioactive labeling were used as substrate source for the unassembled polypeptides. Soluble stroma as well as membrane preparations from isolated plastids contained proteolytic activities catalyzing the degradation of both the small subunits of ribulose‐1,5‐bisphosphate carboxylase and CF 1 ‐δ in vitro . Maximal in vitro degradation was observed at pH 2–3 for the unassembled small subunits, but at pH 6–7 for the purified holoprotein of ribulose‐1,5‐bisphosphate carboxylase, and at pH 6.0 for unassembled CF 1 ‐δ. Degradation of unassembled small subunits of ribulose‐1,5‐bisphosphate carboxylase at pH 3.0 was stimulated by Cu 2+ but not by Ca 2+ , Mg 2+ or ATP. At pH 3.0 the degradation of unassembled small subunits of ribulose‐1,5‐bisphosphate carboxylase was not inhibited by various protease inhibitors but was even stimulated. At pH 7.0 its degradation was inhibited by HgCl 2 and diazoacetyl nor‐leucine methyl ester + Cu‐acetate. The degradation of CF 1 ‐δ was markedly inhibited by phenylmethylsulphonyl fluoride (PMSF) and to a lesser extent by 1,10‐phenanthroline. According to present results different proteolytic systems appear to be involved in the degradation of unassembled small subunits of ribulose‐1,5‐bisphosphate carboxylase and of unassembled CF 1 ‐δ.