The singular properties of photosynthetic cytochrome c from the diatom Phaeodactylum tricornutum suggest new alternative functions.

Cytochrome c is an extrinsic component in the luminal side of photosystem II (PSII) in cyanobacteria, as well as in eukaryotic algae from the red photosynthetic lineage including, among others, diatoms. We have established that cytochrome c from the diatom Phaeodactylum tricornutum can be obtained as a complete protein from the membrane fraction of the alga, although a C-terminal truncated form is purified from the soluble fractions of this diatom as well as from other eukaryotic algae. Eukaryotic cytochromes c show distinctive electrostatic features as compared with cyanobacterial cytochrome c . In addition, co-immunoseparation and mass spectrometry experiments, as well as immunoelectron microscopy analyses, indicate that although cytochrome c from P. tricornutum is mainly located in the thylakoid domain of the chloroplast - where it interacts with PSII - , it can also be found in the chloroplast pyrenoid, related with proteins linked to the CO concentrating mechanism and assimilation. These results thus suggest new alternative functions of this heme protein in eukaryotes.