Induction of acclimative proteolysis of the light-harvesting chlorophyll a/b protein of photosystem II in response to elevated light intensities

摘要

Most plants have the ability to respond to fluctuations in light to minimize damage to the photosynthetic apparatus. A proteolytic activity was discovered in spinach that is involved in the degradation of the major light-harvesting chlorophyll a/b-binding protein of photosystem II (LHCII) when the antenna size of photosystem II is reduced upon acclimatization of plants from low to high light intensities. This ATP-dependent proteolytic activity is of the serine or cysteine type and is associated with the outer membrane surface of the stroma-exposed thylakoid regions. The identity of the protease is not known, but it does not correspond to the recently identified chloroplast ATP-dependent proteases Clp and FtsH, which are homologues to bacterial enzymes. The acclimatization response showed a delay of 2 days after transfer of the leaves to high light. This lag period was shown to be attributed to expression or activation of the responsible protease. Furthermore, the LHCII degradation was regulated at the substrate level. The degradation process involves lateral migration of LHCII from the appressed to the non-appressed thylakoid regions, which is the location for the responsible protease. Phosphorylated LHCII was a poor substrate for degradation incomparison with the unphosphorylated form of the protein.