The reaction specificities of the thylakoidal processing peptidase and Escherichia coli leader peptidase are identical

Proteins which are transported across the bacterial plasma membrane, endoplasmic reticulum and thylakoid membrane are usually synthesized as larger precursors containing amino-terminal targeting signals. Removal of the signals is carried out by specific, membrane-bound processing peptidases. In this report we show that the reaction specificities of these three peptidases are essentially identical. Precursors of two higher plant thylakoid lumen proteins are efficiently processed by purified Escherichia coli leader peptidase. Processing of one precursor, that of the 23 kd photosystem II protein, by both the thylakoidal and E. coli enzymes generates the correct mature amino terminus. Similarly, leader (signal) peptides of both eukaryotic and prokaryotic origin are cleaved by partially purified thylakoidal processing peptidase. No evidence of incorrect processing was obtained. Both leader peptidase and thylakoidal peptidase are inhibited by a synthetic leader peptide.

Endopeptidases in the stroma and thylakoids of pea chloroplasts

Three endopeptidases (EP2, EP3, and EP4) were identified after fractionation of pea (Pisum sativum, var Feltham First) chloroplast stromal extracts. All three were identified by their ability to cleave in vitro-synthesized preplastocyanin to lower molecular weight forms. EP2 is inhibited by phenylmethylsulfonylfluoride, and both EP2 and EP3 are inhibited by the heavy metal chelators 1,10-phenanthroline and EDTA. A further endopeptidase, EP5, was identified in Triton X-100 extracts of thylakoid membranes. Experiments involving contrifugation through a sucrose pad indicate that EP5 either has a high molecular weight or is associated with a thylakoid protein complex. EP5 is effectively inhibited by phenylmethylsulfonylfluoride, iodoacetate, and 1,10-phenanthroline, but not by EGTA. The implications of these results for the analysis of chloroplast protein maturation are discussed, and an improved protocol for the purification of the stromal processing peptidase is described which ensures the removal of EP2, the most active of the stromal peptidases analysed in this study.

ATP-dependent import of a lumenal protein by isolated thylakoid vesicles

The 33 kd protein of the photosynthetic oxygen-evolving complex is synthesized in the cytoplasm as a larger precursor and transported into the thylakoid lumen via a stromal intermediate form. In this report we describe a reconstituted system in which the later stages of this import pathway can be studied in isolation. We demonstrate import of the 33 kd protein, probably as the intermediate form, into isolated pea thylakoids by a mechanism which is stimulated by the addition of ATP. The imported protein is processed to the mature size and is resistant to digestion by proteases. The thylakoidal protein transport system is specific in that non-chloroplast proteins and precursors of stromal proteins are not imported.

Transport of proteins into chloroplasts. Organization, orientation, and lateral distribution of the plastocyanin processing peptidase in the thylakoid network

Plastocyanin is synthesized in the cytoplasm as a larger precursor and transported into the thylakoid lumen of the chloroplast. Maturation of preplastocyanin involves successive cleavages by a stromal peptidase and a distinct thylakoidal peptidase. In this report we have analyzed the precise location and orientation of the thylakoidal peptidase with respect to the thylakoid membrane. Experiments involving differential centrifugation of thylakoid extracts and sonication of isolated vesicles indicate that the peptidase is tightly bound to the thylakoid membrane but not intimately associated with any of the major thylakoid protein complexes. Analysis of the lateral distribution of the peptidase has shown that the enzyme is exclusively located in the non-appressed lamellae of the thylakoid network. The active site of the peptidase is on the lumenal face of the thylakoid membrane.

Transport of proteins into chloroplasts. Partial purification of a thylakoidal processing peptidase involved in plastocyanin biogenesis

Plastocyanin is synthesized in the cytoplasm as a larger precursor and transported across three membranes into the chloroplast thylakoid lumen. Processing to the mature size involves successive cleavages by a stromal and a thylakoidal peptidase. In this report we describe the partial purification and characterization of the thylakoidal peptidase involved. The enzyme has been purified 36-fold from Pisum sativum thylakoids after solubilization using Triton X-100. The peptidase processes the plastocyanin import intermediate to the mature size, but no further, and is capable of processing pre-plastocyanin to the mature size but at a lower rate. No detectable activity is displayed against non-chloroplast proteins or precursors of stromal proteins. The enzyme has a pH optimum of 6.5-7 and is activated by chelating agents such as EDTA and EGTA. No inhibitors of the peptidase have been found to date.