The transit peptide of a chloroplast thylakoid membrane protein is functionally equivalent to a stromal-targeting sequence.
The role of transit peptides in intraorganellar targeting has been studied for a chlorophyll a/b binding (CAB) polypeptide of photosystem II (PSII) and the small subunit of ribulose-1,5-bisphosphate carboxylase (RBCS) from Pisum sativum (pea). These studies have involved in vitro import of fusion proteins into isolated pea chloroplasts. Fusion of the CAB transit peptide to RBCS mediates import to the stroma, as evidenced by assembly of RBCS with chloroplast-synthesized large subunit (RBCL) to form holoenzyme. Similarly, fusion of the RBCS transit peptide to the mature CAB polypeptide mediates import and results in integration of the processed CAB protein into the thylakoid membrane. Correct integration was indicated by association with PSII and assembly with chlorophyll to form the light-harvesting chlorophyll a/b protein complex (LHCII). We interpret these results as evidence that the CAB transit peptide is functionally equivalent to a stromal-targeting sequence and that intraorganellar sorting of the CAB protein must be determined by sequences residing within the mature protein. Our results and those of others suggest that import and integration of CAB polypeptides into the thylakoid proceeds via the stroma.
Hand
J M
JM
Department of Biology, University of Pennsylvania, Philadelphia 19104.
Szabo
L J
LJ
Vasconcelos
A C
AC
Cashmore
A R
AR
D001692:Biological Transport; D002458:Cell Fractionation; D002734:Chlorophyll / Q000378:metabolism*; D002736:Chloroplasts / Q000378:metabolism*; D003001:Cloning, Molecular; D004591:Electrophoresis, Polyacrylamide Gel; D007887:Fabaceae; D007425:Intracellular Membranes / Q000378:metabolism; D045342:Light-Harvesting Protein Complexes; D010455:Peptides / Q000378:metabolism*; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D010946:Plants, Medicinal; D010957:Plasmids; D011498:Protein Precursors / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012273:Ribulose-Bisphosphate Carboxylase / Q000378:metabolism*; D013997:Time Factors
D045342:Light-Harvesting Protein Complexes; D010455:Peptides; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D011498:Protein Precursors; D011993:Recombinant Fusion Proteins; D002734:Chlorophyll; D012273:Ribulose-Bisphosphate Carboxylase