Cytochrome f from the Antarctic green alga Chlamydomonas raudensis: Structure, function, accumulation and assembly into the cytochrome b(6)/f complex

The psychrophillic green alga Chlamydomonas raudensis was isolated from an Antarctic lake and is a natural state transition mutant. Despite the 79% identity in amino acid sequence, cytochrome f (cytf) from C. raudensis has an apparent molecular mass that is 7kD lower than cytf (41kD) from C. reinhardtii. Although the other components of the cyt b6/f complex tested are the same, similar reductions in the apparent molecular mass were observed for the Rubisco large subunit, CP43 and beta-ATPase. The thermostability of heme of cytf from C. raudensis is lower than that of C. reinhardtii. However, we show that the N-terminal heme binding domain of cytf from C. raudensis is not responsible for lower thermostability. Rather the conformation of entire polypeptide and/or oxidation-reduction state of cytf affects the observed differences in thermostability of heme.; The expression of cytf from C. raudensis in C. reinhardtii DeltapetA showed that the apparent molecular mass of cytf in the transformants is identical to that of C. raudensis. Although these transformations did not affect the ability of the C. reinhardtii transformants to undergo state transitions, a reduced capacity for intersystem electron transport compared to C. reinhardtii wild type was observed.; The substitution of only one amino acid in cytf from C. raudensis can modulate the migration rates of this protein on SDS-PAGE. Moreover, domain swapping of cytf suggests that the small domain of this protein is mostly responsible for the differences in conformational states of cytf from both algae. In addition, cytf from C. raudensis expressed in C. reinhardtii exhibited lower abundance when compared to wild type cells. We demonstrate that neither the instability of this protein in thylakoid membranes nor lower transcription level of the petA gene result in lower abundance of cytf from C. raudensis in C. reinhardtii transformants.; We conclude that the subtle differences in amino acid sequence of cyt f from C. raudensis result in specific conformational state causing lower apparent molecular mass and the defective assembly into the cytb6/f complex in C. reinhardtii transformants leading to the degradation of unassembled cyt f. Alternatively, the heme ligation system of C. reinhardtii might be not efficient for the maturation of cytf from C. raudensis, resulting in the degradation of immature protein.