R-phycoerythrin from Gracilaria chilensis:: Estability and interaction among subunits.

Numerous biological macromolecules are arranged in complex interaction systems, to perform their biological function. Phycobiliproteins, the main polypeptidic components of the phycobilisomes, are very good examples for explaining these interactions because their light harvesting and conduction function depend strongly on the three dimensional arrangement of the complex. The functional unit of R-phycoerythrin from Gracilaria chilensis is (alpha beta)(6) and all the subunits require to be perfectly packed to perform the function. For this reason it was selected to study the rol of the different type of interactions in the stability of the protein. To distinguish the contribution of some of the different type of interactions, differential spectrocopy was used to follow the effect of temperature, ionic strength and presence of urea and the structural information was used to analyse the interaction surfaces that are produced during the association. Our results suggest that (alpha beta) is the minimum specie detectable because its interaction surface presents an important hydrophobic component and because in this process, 9 new hydrogen bonds are formed, that contribute to the stability. The stabilization of (alpha beta)(6), requires the contribution of all the subunits involved.