Processing and degradation of chloroplast extension peptides

Abstract

Most chloroplast proteins are synthesized as larger precursors with cleavable extension peptides. These extensions include import signals called transit peptides, export signals for thylakoid transfer, and the C-terminal extension of the chloroplast-encoded D1 subunit of the photosystem II. Transit peptides are necessary for transport of nuclear-encoded proteins from the cytoplasm across the double-membrane envelope, and are cleaved off by Stromal Processing Peptidase (SPP) in the stroma. Further degradation of transit peptides involves SPP and Presequence Protease (PreP). Thylakoid-transfer sequences are required for correct intraorganellar protein sorting and cleaved by Thylakoidal Processing Peptidase (TPP) in the thylakoid lumen. The C-terminal extension of the D1 protein is not required for precursor targeting and integration into the protein complex; however its removal by Carboxyl-terminal peptidase called CtpA in the thylakoid lumen is needed for proper formation of the photosystem II Mn4CaO5 cluster. Biochemical studies in the 1980s-1990s defined basic properties of SPP, TPP and CtpA, while PreP was discovered in the early 2000s. Recent molecular genetic studies demonstrated physiological importance as well as some unprecedented functions of these enzymes. This chapter gives a comprehensive survey on processing and degradation of chloroplast extension peptides. The emphasis is on biochemical, molecular and evolutionary aspects of proteases. The significances of the presence and processing of these extension peptides are also discussed.