Phycobiliprotein diffusion in chloroplasts of cryptophyte Rhodomonas CS24

Unicellular cryptophyte algae employ antenna proteins with phycobilin chromophores in their photosynthetic machinery. The mechanism of light harvesting in these organisms is significantly different than the energy funneling processes in phycobilisomes utilized by cyanobacteria and red algae. One of the most striking features of cryptophytes is the location of the water-soluble phycobiliproteins, which are contained within the intrathylakoid spaces and are not on the stromal side of the lamellae as in the red algae and cyanobacteria. Studies of mobility of phycobiliproteins at the lumenal side of the thylakoid membranes and how their diffusional behavior may influence the energy funneling steps in light harvesting are reported. Confocal microscopy and fluorescence recovery after photobleaching (FRAP) are used to measure the diffusion coefficient of phycoerythrin 545 (PE545), the primary light harvesting protein of Rhodomonas CS24, in vivo. It is concluded that the diffusion of PE545 in the lumen is inhibited, suggesting possible membrane association or aggregation as a potential source of mobility hindrance.