Septins are structural proteins that normally present GTPase activity, playing an important role in the cell structure and acting as a scaffold in the recruitment of partner proteins. In order to do so, different septins organize into heterofilaments that are stabilised by interactions between subunits that show two types of interface, known as G and NC interfaces. Understanding the exact molecular mechanisms that control the correct polymerization of the individual filaments and their later packing into high order structures is one of the biggest challenges in the area of septins' biochemistry. For this purpose, during the last few years, the IFSC - Sao Carlos crystallography group has been working with human septins, both individual such as in physiologically relevant combinations. On the other hand, the best known biological model is not human but the Saccharomyces cerevisiae one, where the septins were originally described and where a huge amount of data concerning physiological characteristics exists. However, structural information related to yeast septins are much more limited. Although the protein Cdc11 has already been purified, crystallized and had its structure reported (Brausemann et al, 2016), many aspects of the structure are unexpected, originating doubts about its reliability. Amongst other things, the stereochemical and crystallographic parameters differ significantly from the norm and a different 3D structure for a highly conserved region (SUE - Septin Unique Element) raises suspicions. For this reason, this project intends, to re-determine the structure of Cdc11 and, subsequently, to extend the structural studies to the other yeast septins. The capacity of reproduce an experiment that generated doubtful data is crucial in science to prove its validity. Also, this activity is fundamental to avoid that a possible error be propagated and become a dubious foundation for future research in the field. The analysis of the structures will be done according to the context already related in the literature (Brausemann et al, 2016), and correlated with other biochemical data, including Cdc11' s ability to bind (or not) and hydrolyse GTP. The structure of Cdc11 will also allow a full description of the nature of its NC interface, essential for the filament polymerization. The acquired knowledge will contribute to an understanding of the correlation between structural and functional aspects of septins in yeast, considered to this 'day to be a neglected element of the cytoskeleton. . '
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