title: The sec-independent twin-arginine translocation system can transport both tightly folded and malfolded proteins across the thylakoid membrane.
abstract: A subset of lumen proteins is transported across the thylakoid membrane by a Sec-independent translocase that recognizes a twin-arginine motif in the targeting signal. A related system operates in bacteria, apparently for the export of redox cofactor-containing proteins. In this report we describe a key feature of this system, the ability to transport folded proteins. The thylakoidal system is able to transport dihydrofolate reductase (DHFR) when an appropriate signal is attached, and the transport efficiency is almost undiminished by the binding of folate analogs such as methotrexate that cause the protein to fold very tightly. The system is moreover able to transport DHFR into the lumen with methotrexate bound in the active site, demonstrating that the DeltapH-driven transport of large, native structures is possible by this pathway. However, correct folding is not a prerequisite for transport. Truncated, malfolded DHFR can be translocated by this system, as can physiological substrates that are severely malfolded by the incorporation of amino acid analogs.
authors.affiliation: Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, United Kingdom.
authors.forename: P J
authors.initials: PJ
authors.lastname: Hynds
authors.forename: D
authors.initials: D
authors.lastname: Robinson
authors.forename: C
authors.initials: C
authors.lastname: Robinson
mesh_terms: D000630:Aminopterin / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D002352:Carrier Proteins / Q000378:metabolism*; D002451:Cell Compartmentation; D002736:Chloroplasts / Q000378:metabolism*; D005493:Folic Acid Antagonists / Q000494:pharmacology; D007425:Intracellular Membranes / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism; D008727:Methotrexate / Q000494:pharmacology; D018532:Pisum sativum; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D017510:Protein Folding; D011498:Protein Precursors / Q000235:genetics / Q000378:metabolism; D021382:Protein Sorting Signals / Q000378:metabolism; D018892:Proton-Motive Force; D011994:Recombinant Proteins / Q000378:metabolism; D000069816:SEC Translocation Channels; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics / Q000378:metabolism
chemical_list: D002352:Carrier Proteins; D005493:Folic Acid Antagonists; D008565:Membrane Proteins; D010940:Plant Proteins; D011498:Protein Precursors; D021382:Protein Sorting Signals; D011994:Recombinant Proteins; D000069816:SEC Translocation Channels; D013762:Tetrahydrofolate Dehydrogenase; D000630:Aminopterin; D008727:Methotrexate