FtsH Protease Inactivation Allows Accumulation of Aberrant Photosystem II in a Chlamydomonas Rubredoxin Mutant

The assembly of photosystem II (PSII) requires the participation of assembly proteins that facilitate the step-wise association of its protein and pigment components into a functional complex capable of oxidizing water and reducing plastoquinone. We previously identified one such factor, the membrane-bound rubredoxin RBD1, but its precise role remains unknown in part due to the inability of the 2pac mutant strain of Chlamydomonas reinhardtii, which lacks RBD1, to accumulate PSII. Here, we show that decreased PSII accumulation in 2pac is due to increased proteolytic degradation. Inactivating the thylakoid membrane FtsH protease in the 2pac mutant background led to an increase in the abundance of PSII subunits and their integration into higher molecular weight complexes, including PSII dimers, capable of sustaining photoautotrophic growth. Dark- and low light-grown 2pac ftsh1-1 both accumulated a 23-kD fragment of the D1 protein, a marker typically associated with structural changes resulting from photodamage or photoinhibition. We introduced a HIS-tagged version of the PsbH protein into the 2pac ftsh1-1 background to purify and examine PSII. We found no detectable changes with respect to cofactor composition relative to the wild-type, leading to us to propose a model in which RBD1 promotes the proper folding of D1, possibly via delivery or reduction of the non-heme iron during PSII assembly. Our results demonstrate that introduction of the ftsh1-1 mutation into mutants defective in the biogenesis of thylakoid membrane complexes can allow for the accumulation and study of aberrant complexes that would otherwise be degraded due to their high protease sensitivity.