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Published online 17 October 2005. doi:10.1083/jcb.200502067
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 171, Number 2, 281-289
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Article

 Large-scale translocation reversal within the thylakoid Tat system in vivo

Alessandra Di Cola and Colin Robinson

Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, England, UK

Correspondence to Colin Robinson: crobinson{at}bio.warwick.ac.uk

In vitro import assays have shown that the thylakoid twin-arginine translocase (Tat) system transports folded proteins in a unidirectional manner. Here, we expressed a natural substrate, pre-23K, and a 23K presequence–green fluorescent protein (GFP) chimera in vivo in tobacco protoplasts. Both are imported into chloroplasts, targeted to the thylakoids, and processed to the mature size by the lumen-facing processing peptidase. However, the vast majority of mature GFP and about half of the 23K are then returned to the stroma. Mutations in the twin-arginine motif block thylakoid targeting and maturation, confirming an involvement of the Tat apparatus. Mutation of the processing site yields membrane-associated intermediate-size protein in vivo, indicating a delayed reversal of translocation to the stroma and suggesting a longer lived interaction with the Tat machinery. We conclude that, in vivo, the Tat system can reject substrates at a late stage in translocation and on a very large scale, indicating the influence of factors that are absent in reconstitution assays.

Abbreviations used in this paper: DP, degradation product; iGFP, intermediate GFP; Tat, twin-arginine translocase; TPP, thylakoidal processing peptidase.


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