A key piece of the puzzle: The central tetramer of the Saccharomyces cerevisiae septin protofilament and its implications for self-assembly.

Septins, often described as the fourth component of the cytoskeleton, are structural proteins found in a vast variety of living beings. They are related to small GTPases and thus, generally, present GTPase activity which may play an important (although incompletely understood) role in their organization and function. Septins polymerize into long non-polar filaments, in which each subunit interacts with two others by alternating interfaces, NC and G. In Saccharomyces cerevisiae four septins are organized in the following manner, [Cdc11-Cdc12-Cdc3-Cdc10-Cdc10-Cdc3-Cdc12-Cdc11]n in order to form filaments. Although septins were originally discovered in yeast and much is known regarding their biochemistry and function, only limited structural information about them is currently available. Here we present crystal structures of Cdc3/Cdc10 which provide the first view of the physiological interfaces formed by yeast septins. The G-interface has properties which place it in between that formed by SEPT2/SEPT6 and SEPT7/SEPT3 in human filaments. Switch I from Cdc10 contributes significantly to the interface, whereas in Cdc3 it is largely disorded. However, the significant negative charge density of the latter suggests it may have a unique role. At the NC-interface, we describe an elegant means by which the sidechain of a glutamine from helix α0 imitates a peptide group in order to retain hydrogen-bond continuity at the kink between helices α5 and α6 in the neighbouring subunit, thereby justifying the conservation of the helical distortion. Its absence from Cdc11, along with this structure's other unusual features are critically discussed by comparison with Cdc3 and Cdc10.

Unexpected plasticity of the quaternary structure of iron-manganese superoxide dismutases.

Protein 3D structure can be remarkably robust to the accumulation of mutations during evolution. On the other hand, sometimes a single amino acid substitution can be sufficient to generate dramatic and completely unpredictable structural consequences. In an attempt to rationally alter the preferences for the metal ion at the active site of a member of the Iron/Manganese superoxide dismutase family, two examples of the latter phenomenon were identified. Site directed mutants of SOD from Trichoderma reesei were generated and studied crystallographically together with the wild type enzyme. Despite being chosen for their potential impact on the redox potential of the metal, two of the mutations (D150G and G73A) in fact resulted in significant alterations to the protein quaternary structure. The D150G mutant presented alternative inter-subunit contacts leading to a loss of symmetry of the wild type tetramer, whereas the G73A mutation transformed the tetramer into an octamer despite not participating directly in any of the inter-subunit interfaces. We conclude that there is considerable intrinsic plasticity in the Fe/MnSOD fold that can be unpredictably affected by single amino acid substitutions. In much the same way as phenotypic defects at the organism level can reveal much about normal function, so too can such mutations teach us much about the subtleties of protein structure.