Abstract
Light-harvesting chlorophyll a/b-binding protein, LHCP, or its precursor, pLHCP, cannot be stably inserted into barley etioplast membranes in vitro. However, when these etioplast membranes are supplemented with the chlorophyll analogs Zn-pheophytin a/b, synthesized in situ from Zn-pheophorbide a/b and digeranyl pyrophosphate, pLHCP is inserted into a protease-resistant state. This proves that chlorophyll is the only component lacking in etioplast membranes that is necessary for stable LHCP insertion. Synthesis of Zn-pheophytin b alone promotes insertion of LHCP in vitro into a protease-resistant state, whereas synthesis of Zn-pheophytin a alone does not. Insertion of pLHCP into etioplast membranes can also be stimulated by adding chlorophyll a and chlorophyll b to the membranes, albeit at a significantly lower efficiency as compared with Zn-pheophytin a/b synthesized in situ. When pLHCP is inserted into chlorophyll- or Zn-pheophytin-supplemented etioplast membranes and then assayed with protease, only the protease digestion product indicative of the monomeric major light-harvesting chlorophyll a/b complex (LHCII) is found but not the one indicating trimeric complexes. In this respect, chlorophyll- or Zn-pheophytin-supplemented etioplast membranes resemble thylakoid membranes at an early greening stage: pLHCP inserted into plastid membranes from greening barley is assembled into trimeric LHCII only after more than 1 h of greening.
Highlights
The major light-harvesting chlorophyll a/b complex (LHCII)1 of photosystem II is the predominant pigment-protein complex in higher plants, comprising roughly half of the total chlorophyll (Chl) in plant cells
In intermittent light (IML)grown plants containing little or no Chl b, no LHCII apoprotein (LHCP) is accumulated in the membrane, presumably translatable mRNA is present in normal amounts
Stable Insertion of pLHCP in Etioplast Membranes Is Stimulated by Synthesis of Chl a and Chl b in Situ—Isolated thylakoid membranes are capable of stably inserting LHCP in vitro, rendering the protein largely protease-resistant
Summary
The major light-harvesting chlorophyll a/b complex (LHCII) of photosystem II is the predominant pigment-protein complex in higher plants, comprising roughly half of the total chlorophyll (Chl) in plant cells. In intermittent light (IML)grown plants containing little or no Chl b, no LHCP is accumulated in the membrane, presumably translatable mRNA is present in normal amounts. This observation led to the concept that stabilization by pigments of LHCP in the thylakoid is part of the post-translational regulation ensuring the coordinate accumulation of light-harvesting protein and light-harvesting pigments; excess protein that is not complexed with pigments is rapidly degraded by plastid proteases [6, 7]. With the pigment requirement observed in vivo, etioplast membranes do not accumulate inserted LHCP even when they are supplemented with a stromal fraction from greening chloroplasts [11]. PLHCP Insertion into Zn-phe-supplemented Etioplast Membranes tion, and we have shown previously that pLHCP becomes inserted into isolated thylakoids and assembled into LHCII trimers with comparable efficiency as the mature protein [17]
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