Wednesday, 12 November 2014

Compartmentalization of the protein repair machinery in photosynthetic membranes

http://www.pnas.org/content/111/44/15839.full

Sujith Puthiyaveetil, Onie Tsabari, Troy Lowry, Steven Lenhery, Robert R. Lewis, Ziv Reich and Helmut Kirchoff

Photosynthesis relies on the functioning of the PSII complex, which is embedded in the thylakoid membranes of plant chloroplasts. This complex is large and composed of multiple subunits, of which the D1 subunit bears most of the brunt of the damage from photosynthesis. The complex undergoes a repair cycle which reduces the amount of protein synthesis required by only replacing the D1 subunit through a series of reactions designed to remove and degrade the damaged D1 subunit without destroying the entire complex, before synthesising a new D1 subunit. This repair process occurs in the stroma lamellae (which connect the stacked grana), however PSII is concentrated in the stacked grana. PSII phosphorylation triggers its disassembly before it is transported through the grana margin to the stroma lamellae for D1 replacement. The repair cycle is involved and must happen rapidly - the entire PSII complement of a plant can be turned over in just 1 hour. This requires an efficient process with minimal back-reactions.

The authors investigated the organisation of the PSII repair system through fractionation of thylakoid membranes and electron microscopy (EM) investigation of stacked grana. High-light (HL) treatment, which causes increased damage to PSII, led to growth in grana margins at the expense of grana core and stroma lamellae. By quantifying levels of key proteins involved in PSII repair in the grana core, margins and stroma lamellae, the authors deduced that localising different components in different areas allows the PSII repair system to improve its throughput by minimising back-reactions. In addition, localisation of proteolytic steps also reduces the amount of unnecessary protein degradation, by ensuring that access to proteasomes is restricted.

The authors identify discovering factors that govern the localisation of enzymes to the appropriate compartment as an important future step.

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