THE PLANT CELL, Vol 1, Issue 1 159-166, Copyright © 1989 by American Society of Plant Biologists
A Hydrophobic, Carboxy-Proximal Region of a Light-Harvesting Chlorophyll a/b Protein Is Necessary for Stable Integration into Thylakoid Membranes
B. D. Kohorn and E. M. Tobin
Department of Botany, Duke University, Durham, North Carolina 27706
Proteins synthesized as soluble precursors in the cytoplasm of eukaryotic
cells often cross organellar membrane barriers and then insert into lipid
bilayers. One such polypeptide, the light-harvesting chlorophyll
a/b-binding protein (LHCP), must also associate with pigment molecules and
be assembled into the photosystem II light-harvesting complex in the
chloroplast thylakoid membrane. A study of the import of mutant LHCPs into
isolated chloroplasts has shown that a putative [alpha]-helical
membrane-spanning domain near the carboxy terminus (helix 3) is essential
for the stable insertion of LHCP in the thylakoid. Protease digestion
experiments are consistent with the carboxy terminus of the protein being
in the lumen. This report also shows that helix 3, when fused to a soluble
protein, can target it to the thylakoids of isolated, intact chloroplasts.
Although helix 3 is required for the insertion of LHCP and mutant
derivatives into the thylakoid, the full insertion of helix 3 itself
requires additionally the presence of other regions of LHCP. Thus, LHCP
targeting and integration into thylakoid membranes requires a complex
interaction involving a number of different domains of the LHCP
polypeptide.
