Phosphatidylglycerol is involved in the dimerization of photosystem II

Photosystem II core dimers (450 kDa) and monomers (230 kDa) consisting of C P47, CP43, the D1 and D2 proteins, the extrinsic 33-kDa subunit, and the lo w molecular weight polypeptides PsbE, PsbF, PsbH, PsbI, PsbK, PsbL, PshTc, and PsbW were isolated by sucrose density gradient centrifugation. The phot osystem II core dimers were treated with phospholipase A2 (PL-A2), which cu ts phosphatidylglycerol (PG) and phosphatidylcholine molecules at the sn-2 position. The PL-A2-treated dimers dissociated into two core monomers and f urther, yielding a CP47-D1-D2 subcomplex and CP43. Thin layer chromatograph y showed that photosystem II dimers contained four times more PG than their monomeric counterparts but with similar levels of phosphatidylcholine. Con sistent with this was the finding that, compared with monomers, the dimers contained a higher level of trans-hexadecanoic fatty acid (C16:1 Delta 3tr) , which is specific to PG of the thylakoid membrane. Moreover, treatment of dimers with PL-A2 increased the free level of this fatty acid specific to PG compared with untreated dimers. Further evidence that PG is involved in stabilizing the dimeric state of photosystem II comes from reconstitution e xperiments. Using size exclusion chromatography, it was shown that PG conta ining C16:1 Delta 3tr, but not other lipid classes, induced significant dim erization of isolated photosystem II monomers. Moreover, this dimerization was observed by electron crystallography when monomers were reconstituted i nto thylakoid lipids containing PG. The unit cell parameters, p2 symmetry a xis, and projection map of the reconstituted dimer was similar to that obse rved for two dimensional crystals of the native dimer.