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PLANT PHYSIOLOGY , Vol 102, Issue 1 35-43, Copyright © 1993 by American Society of Plant Biologists
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CELL BIOLOGY AND SIGNAL TRANSDUCTION |
Biogenesis of a Photosystem I Light-Harvesting Complex (Evidence for a Membrane Intermediate)
Z. Adam and N. E. Hoffman
Department of Plant Biology, Carnegie Institution of Washington, Stanford, California 94305
CAB-7p is a chlorophyll a/b binding protein of photosystem I (PSI). It is
found in light-harvesting complex I 680 (LHCI-680), one of the chlorophyll
complexes produced by detergent solubilization of PSI. Two types of
evidence are presented to indicate that assembly of CAB-7p into PSI
proceeds through a membrane inter-mediate. First, when CAB-7p is briefly
imported into chloroplasts or isolated thylakoids, we initially observe a
fast-migrating membrane form of CAB-7p that is subsequently converted into
PSI. The conversion of the fast-migrating form into PSI does not require
stroma or ATP. Second, trypsin treatment of thylakoids containing
radiolabeled CAB-7p indicates that there are at least two membrane forms of
the mature 23-kD protein. The predominant form is completely resistant to
proteolysis; a second form of the protein is cleaved by trypsin into 12-
and 7-kD polypeptides. We interpret this to mean that the intermediate is a
cleavable form that becomes protease resistant during assembly. This notion
is supported by the observation that CAB-7p in LHCI-680 is largely cleaved
by trypsin into 12- and 7-kD polypeptides, whereas CAB-7p in isolated PSI
particles is trypsin resistant. In vitro, we generated a mutant form of
CAB-7p, CAB-7/Bgl2p, that was able to integrate into thylakoid membranes
but was unable to assemble into PSI. The membrane form of CAB-7/Bgl2p, like
LHCI-680, was predominantly cleaved by trypsin into 12- and 7-kD fragments.
We suggest that the mutant protein is arrested at an intermediate stage in
the assembly pathway of PSI. Based on its mobility in nondenaturing gels
and its susceptibility to protease cleavage, we suggest that the
intermediate form is LHCI-680. We propose the following distinct stages in
the bio-genesis of LHCI: (a) apoprotein is integrated into the thylakoid,
(b) chlorophyll is rapidly bound to apoprotein forming LHCI-680, and (c)
LHCI-680 assembles into the native PSI complex.
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