Proteolytic processing of imported chloroplast proteins

• Main Author: Elderfield, Peter D.
• Published: University of Warwick 1990
• Subjects: 572; QK Botany
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.293661

Three proteins located in the thylakoid lumen, plastocyanin and the 23KDa and 33KDa oxygen evolving polypeptides of photosystem II, are synthesised in the cytoplasm as higher molecular weight precursors, with N-terminal transit peptides. Import of these proteins involves removal of the first part of the transit peptide by a stromal processing peptidase to yield an intermediate. Maturation, by removal of the remaining transit peptide is performed by a thylakoidal processing peptidase (TPP). TPP has been partially purified and characterised from pea thylakoids and found to be an integral thylakoid membrane protein with the active site on the lumenal (trans) side of the membrane. TPP has a molecular weight of less than 250 000 and is not associated with any supra-molecular complex. Partial purification has yielded ten bands on a Coomassie stained SDS-PAGE gel; however TPP has not been attributed to any of these bands. TPP displays specificity for chloroplast protein precursors with transit peptides containing a thylakoid transfer domain; however, no species specificity is displayed. TPP exhibits similarities in reaction specificity to Escherichia coli leader peptidase (LEP) in that both peptidases cleave the same eukaryotic and bacterial precursor as well as cleaving higher plant lumenal precursors at the predicted cleavage site. No standard protease inhibitor has been found to abolish TPP activity; however, a synthetic signal sequence polypeptide will inhibit TPP and LEP. A thylakoidal endopeptidase (EPS) has been discovered which cleaves lumenal precursors to a size slightly larger than the mature size. EP5 displays different inhibitor sensitivities to TPP and is either a high molecular weight protein or associated with a supramolecular complex. EPS is assumed to be involved in the turnover of thylakoid proteins.