Structural stability of Brucella spp. lumazine synthase. (Poster)

VANESA ZYLBERMAN; SEBASTIÁN KLINKE; PATRICIO O. CRAIG; BRADFORD C. BRADEN; ANA CAUERHFF; FERNANDO A.GOLDBAUM

Bariloche, Pcia. de Río Negro, Argentina

Congreso; XXXIX Reunión anual de SAIB (Sociedad Argentina de Investigación en Bioquímica y Biología Molecular) y SAB (Sociedad Argentina de Biofísica), Bariloche Protein Symposium.; 2003

SAIB y SAB

The penultimate step in the pathway of riboflavin biosynthesis is catalyzed by the enzyme lumazine synthase (LS). One of the most distinctive characteristics of this enzyme is the structural quaternary divergence found in different species. The protein exists as pentameric and icosahedral forms, built from practically the same structural monomeric unit. The pentameric structure (fungal M. grisea, yeast S. cereviseae and S. pombe) is formed by five monomers each of them (18 kDa) extensively contacts the neighboring monomers. The icosahedrical structure (B. subtilis, A. aelicus and plant spinach) consists of 60 LS monomers arranged as twelve pentamers giving rise to a capsid exhibiting icosahedral 532 symmetry. In all LSs studied, the topologically equivalent active sites are located at the interfaces between adjacent subunits in the pentameric modules. The B. abortus LS (BLS) sequence clearly diverges from pentameric and icosahedric enzymes. This unusual divergence prompted us to further investigate on the quaternary arrangement of BLS. In the present work, we demonstrate by means of solution Light Scattering and x-ray structural analyses that BLS folds as a very stable dimmer of pentamers, representing a third category of quaternary assembly for LSs. We also describe by spectroscopic studies the thermodynamic stability of this oligomeric protein, and postulate a mechanism for dissociation/unfolding of this macromolecular assembly.