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© The Rockefeller University Press, 0021-9525/1999//267 $5.00
The Journal of Cell Biology, Volume 147, Number 2, , 1999 267-276

Original Article

The Maize tha4 Gene Functions in Sec-Independent Protein Transport in Chloroplasts and Is Related to hcf106, tatA, and tatB



Macie B. Walkera, Laura M. Roya, Eric Colemana, Rodger Voelkera, and Alice Barkana

a Institute of Molecular Biology, University of Oregon, Eugene, Oregon 97403
University of Oregon, Institute of Molecular Biology, 1370 Franklin Boulevard, Klamath 297, Eugene, OR 97403.(541) 346-5891(541) 346-5145

abarkan{at}molbio.uoregon.edu

Proteins are translocated across the chloroplast thylakoid membrane by a variety of mechanisms. Some proteins engage a translocation machinery that is derived from the bacterial Sec export system and require an interaction with a chloroplast-localized SecA homologue. Other proteins engage a machinery that is SecA-independent, but requires a transmembrane pH gradient. Recently, a counterpart to this {Delta} pH mechanism was discovered in bacteria. Genetic studies revealed that one maize protein involved in this mechanism, HCF106, is related in both structure and function to the bacterial tatA and tatB gene products. We describe here the mutant phenotype and molecular cloning of a second maize gene that functions in the {Delta} pH mechanism. This gene, thylakoid assembly 4 (tha4), is required specifically for the translocation of proteins that engage the {Delta} pH pathway. The sequence of the tha4 gene product resembles those of the maize hcf106 gene and the bacterial tatA and tatB genes. Sequence comparisons suggest that tha4 more closely resembles tatA, and hcf106 more closely resembles tatB. These findings support the notion that this sec-independent translocation mechanism has been highly conserved during the evolution of eucaryotic organelles from bacterial endosymbionts.

Key Words: protein export • thylakoid • Mutator • maize • Sec-independent

© 1999 The Rockefeller University Press

1.used in this paper: Mu, Mutator; OE16, 16-kD subunit of the oxygen-evolving complex associated with photosystem II; OE23, 23-kD subunit of the oxygen-evolving complex associated with photosystem II; PC, plastocyanin; tat, twin arginine translocation; UTR, untranslated region

L.M. Roy's current address is Department of Biology, University of California, San Diego, CA 92093.

E. Coleman's current address is 345 Cinnamon Drive, Satellite Beach, FL 32937.

R. Voelker's current address is Oregon Department of Agriculture, Salem, OR 97310.


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