THE PLANT CELL, Vol 9, Issue 3 441-452, Copyright © 1997 by American Society of Plant Biologists
Assembly of Newly Imported Oxygen-Evolving Complex Subunits in Isolated Chloroplasts: Sites of Assembly and Mechanism of Binding
A. Hashimoto, W. F. Ettinger, Y. Yamamoto and S. M. Theg
Section of Plant Biology, University of California at Davis, Davis, California 95616
We have examined the assembly of the nuclear-encoded subunits of the
oxygen-evolving complex (OEC) after their import into isolated intact
chloroplasts. We showed that all three subunits examined (OE33, OE23, and
OE17) partition between the thylakoid lumen and a site on the inner surface
of the thylakoid membrane after import in a homologous system (e.g., pea or
spinach subunits into pea or spinach chloroplasts, respectively). Although
some interspecies protein import experiments resulted in OEC subunit
binding, maize OE17 did not bind thylakoid membranes in chloroplasts
isolated from peas. Newly imported OE33 and OE23 were washed from the
membranes at the same concentrations of urea and NaCl as the native,
indigenous proteins; this observation suggests that the former subunits are
bound productively within the OEC. Inhibition of neither chloroplast
protein synthesis nor light- or ATP-dependent energization of the thylakoid
membrane significantly affected these assembly reactions, and we present
evidence suggesting that incoming subunits actively displace those already
bound to the thylakoid membrane. Transport of OE33 took place primarily in
the stromal-exposed membranes and proceeded through a protease-sensitive,
mature intermediate. Initial binding of OE33 to the thylakoid membrane
occurred primarily in the stromal-exposed membranes, from where it migrated
with measurable kinetics to the granal region. In contrast, OE23 assembly
occurred in the granal membrane regions. This information is incorporated
into a model of the stepwise assembly of oxygen-evolving photosystem II.
