Studies on C-phycocyanin from Cyanidium caldarium, a eukaryote at the extremes of habitat

C-Phycocyanin, a biliprotein, was purified from the red alga, Cyanidium cal darium. This alga grows at temperatures up to 57 degrees C, a very high tem perature for a eukaryote, and at pH values down to 0.05. Using the chromoph ores on C-phycocyanin as naturally occurring reporter groups, the effects o f temperature on the stability of the protein were studied by circular dich roism and absorption spectroscopy. The protein was unchanged from 10 to 50 degrees C, which indicates that higher temperatures are not required to cau se the protein to be photosynthetically active. At 60 and 65 degrees C, whi ch are above the temperatures at which the alga can survive, the protein un dergoes irreversible denaturation. Gel-filtration column chromatography dem onstrated that the irreversibility is caused by the dissociation of the tri meric protein to its constitutive polypeptides. Upon cooling, the alpha and beta polypeptides did not reassemble to the trimer. Unlike phycocyanins 64 5 and 612, the C-phycocyanin does not show a reversible conformational chan ge at moderately high temperatures. At constant temperature, the C-phycocya nin was more stable than a mesophilic counterpart. It is designated a tempe rature-resistant protein. (C) 2000 Elsevier Science B.V. All rights reserve d.