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© The Rockefeller University Press, 0021-9525/1998//385 $5.00
The Journal of Cell Biology, Volume 141, Number 2, , 1998 385-395

Articles

A SecY Homologue Is Required for the Elaboration of the Chloroplast Thylakoid Membrane and for Normal Chloroplast Gene Expression



Laura M. Roy and Alice Barkan

Institute of Molecular Biology, University of Oregon, Eugene, Oregon 97403

Results of in vitro and genetic studies have provided evidence for four pathways by which proteins are targeted to the chloroplast thylakoid membrane. Although these pathways are initially engaged by distinct substrates and involve some distinct components, an unresolved issue has been whether multiple pathways converge on a common translocation pore in the membrane. A homologue of eubacterial SecY called cpSecY is localized to the thylakoid membrane. Since SecY is a component of a protein-translocating pore in bacteria, cpSecY likely plays an analogous role. To explore the role of cpSecY, we obtained maize mutants with transposon insertions in the corresponding gene. Null cpSecY mutants exhibit a severe loss of thylakoid membrane, differing in this regard from mutants lacking cpSecA. Therefore, cpSecY function is not limited to a translocation step downstream of cpSecA. The phenotype of cpSecY mutants is also much more pleiotropic than that of double mutants in which both the cpSecA- and {Delta}pH-dependent thylakoid-targeting pathways are disrupted. Therefore, cpSecY function is likely to extend beyond any role it might play in these targeting pathways. CpSecY mutants also exhibit a defect in chloroplast translation, revealing a link between chloroplast membrane biogenesis and chloroplast gene expression.

Abbreviations used in this paper: LHCP, light-harvesting chlorophyll a/b binding protein; OE16, 16-kD subunit of oxygen evolving complex; OE23, 23-kD subunit of oxygen evolving complex; OE33, 33-kD subunit of oxygen evolving complex; PC, plastocyanin; Rubisco, ribulose biphosphate carboxylase; SRP, signal recognition particle; csy1, chloroplast secY 1.

We would like to express our deep gratitude to K. Canada and especially to B. Meeley at Pioneer Hi-Bred for identifying and providing the mutants used in this study, and for being such forthcoming and helpful collaborators. We would also like to thank J. Selker for performing the electron microscopy, Y. Wang for DNA sequencing, R. Voelker and M. Walker for technical advice, and R. Voelker and M. Covington for help with the tha1/tha4 double mutant experiment (all four from University of Oregon, Eugene, OR). Antibodies were provided by S. Merchant (University of California, Los Angeles, CA) (CF1), B. Taylor (CSIRO, Canberra, Australia) (LHCP), D. Malkin (University of California, Berkeley, CA) (PsaF), and K. Keegstra and J. Davila-Aponte (both from Michigan State University, East Lansing, MI) (TIC110). hcf106-mum3 seed was provided by Rob Martienssen (Cold Spring Harbor Laboratory, Cold Spring Harbor, NY).

Address all correspondence to Alice Barkan, Institute of Molecular Biology, University of Oregon, Eugene, OR 97403. Tel.: (541) 346-5145. Fax: (541) 346-5891.


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