A coil-helix instead of a helix-coil motif can be induced in a chloroplasttransit peptide from Chlamydomonas reinhardtii

A synthetic peptide MQVTMKSSAVSGQRVGGARVATRSVRRAQLQV corresponding to the 3 2 amino acid chloroplast transit sequence of the ribulose bisphosphatase ca rboxylase/oxygenase activase preprotein from Chlamydomonas reinhardtii, req uired for translocation through the envelope of the chloroplast, has been c haracterized structurally using CD and NMR under the same experimental cond itions as used previously for the 32 amino acid presequence of preferredoxi n from the same organism [Lancelin, J.-M., Bally, I., Arlaud, G. J., Blackl edge, M., Gans, P., Stein, M. & Jacquot, J.-P. (1994) FEES Lett. 343, 261-2 66]. The peptide is found to undergo a conformational transition in aqueous 2,2,2-trifluoroethano1, characterized by three turns of amphiphilic a-heli x in the C-terminal region preceded by a disordered coil in the N-terminal region. Compared with the preferredoxin transit peptide, the helical and co iled domains are arranged in the reverse order along the peptide sequence, but the positively charged groups are distributed analogously as well as th e hydrophobic residues within the amphiphilic a-helix. It is proposed that such coil-helix or helix-coil motifs, occasionally repeated, could he an in trinsic structural feature of chloroplastic transit peptides, adapted to th e proper translocase and possibly to each nuclear-encoded chloroplast prepr oteins. This feature may distinguish chloroplastic transit sequences from t he other organelle-targeting peptides in the eukaryotic green alga C. reinh ardtii, particularly the mitochondrial transit sequences.