Thermophilic C-phycocyanin: effect of temperature, monomer stability, and structure

Abstract

C-Phycocyanin has been purified from Synechococcus lividus (SyI), which grows between 66 and 73°C at the highest temperature of any cyanobacterium and of any biliprotein-containing organism. The protein was examined at its physiological temperature, 70°C, in order to compare with its properties at 20°C. The protein was found to have virtually identical properties of light harvesting, bilin conformation, fluorescence, and secondary protein structure at both temperatures. The unique absorption maximum of SyI protein was maintained at 70°C. Using the bilins as built-in reporter groups, the response is to resist, over a wide temperature range (10–80°C), the denaturation that would occur in mesophiles, rather than exhibit special adaptation to 70°C. C-Phycocyanin from another thermophile, S. lividus (SyIII), behaved differently. The SyI protein was `temperature-resistant', and the other `cold-dissociated'. In addition, an early assembly step toward the phycobilisome is the formation of monomers (αβ) from bilin-bearing, α and β polypeptides. Unlike larger aggregates, C-phycocyanin monomers from SyI are found to denature between 60°C and 70°C. The instability of monomers at their physiological temperature suggests that they either rapidly aggregate to avoid denaturation, or they are protected from denaturation by some agent. Monomers of the thermophile were, however, much more stable against high temperature than the monomers from mesophiles. The structure of SyI C-phycocyanin and its phycobilisomes were shown to be similar to those of mesophiles.