Practice question

Final intra-chloroplast sorting thylakoid lumen after stroma entry requires crossing thylakoid membrane barrier maintaining proton gradient ATP synthesis. Lumenal precursors plastocyanin copper OE16 OE23 photosystem II oxygen evolving complex Rieske iron-sulfur PetC transport stroma lumen. Precursors carry bipartite targeting sequences N-terminal chloroplast transit peptide TOC-TIC entry second thylakoid transfer domain 30-70 residues positively charged N-region hydrophobic core C-region cleavage thylakoid processing peptidase. Two distinct pathways recognize transfer domain: Sec pathway transports unfolded chains SecYE channel driven SecA ATPase proteins plastocyanin apoprotein folding after transport Tat twin-arginine translocation transports fully folded proteins already assembling cofactors disulfide bonds stroma cofactor insertion before transport essential unfolded impossible. Diagnostic signature Tat substrates twin-arginine motif SRRXFLK invariant consecutive arginines RR N-region signal plus hydrophobic Phe-Leu-Lys essential recognition cpTatC Hcf106 receptor complex oligomeric pore varied size driven exclusively proton motive force Delta pH across thylakoid not ATP. SKL peroxisomal PTS1 matrix NPXY endocytic internalization PTB adaptors DXE ER export COPII unrelated thylakoid lumen sorting. Mutation arginine pair lysine abolishes import demonstrating motif necessity and energy coupling mechanism distinct.