Protein Targeting and Vesicular Trafficking
Dabney-Smith, Carole , Cline, Kenneth C. .
The role of Tha4 oligomers as the protein conducting component for folded protein transport by the chloroplast twin arginine transport system.
The chloroplast Tat pathway uses three known membrane proteins—cpTatC, Hcf106, and Tha4 and the proton gradient to transport folded proteins across the thylakoid membrane. cpTatC and Hcf106 form a large complex that serves as the signal peptide receptor. Tha4 assembles with the precursor-receptor complex as a required prelude to the translocation step. These observations and the large molar excess of Tha4 over cpTatC and Hcf106 led to models for protein translocation where Tha4 oligomers form the protein-conducting channel. One model suggests that Tha4 channels exist as preformed oligomers of varying sizes to fit different precursors. Another model suggests Tha4 assembles according to the size of the precursor. Tha4 is a single-span membrane protein with an adjacent surface-active amphipathic helix and an unstructured C-tail. The role of Tha4 oligomers was addressed with isolated thylakoids, a functional Tha4 replacement assay, and a cysteine scanning/oxidative disulfide crosslinking method. Tha4 forms oligomers in the absence of protein transport via its transmembrane domain. However, when activated for protein transport, Tha4 oligomerizes through its C-tail to form homo-oligomers of at least 14 protomers. Amazingly, the same size oligomer, as determined by gradient gel SDS-PAGE, was stimulated by a 3 kD signal peptide lacking the mature domain as by a 23 kD precursor. These results favor a model where Tha4 does not exist as preformed oligomers of varying size, nor does it size the precursor by building a channel on demand. Rather, we suggest a novel mechanism whereby Tha4 oligomers recruited to the precursor bound receptor serve as a pressure-sensitive trap-door, folding in their amphipathic helices to facilitate movement of the precursor across the membrane bilayer.
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1 - University of Florida, Horticultural Sciences Dept. and Plant Molecular and Cellular Biology Program, Box 110690, Gainesville, FL, 32611, USA
twin arginine transport
chloroplast protein transport
Presentation Type: Plant Biology Abstract
Location: Exhibit Hall (Northeast, Southwest & Southeast)/Hilton
Date: Sunday, July 8th, 2007
Time: 8:00 AM