Kapłon Tomasz M, Michnik Anna, Drzazga Zofia, Richter Klaus, Kochman Marian, Ozyhar Andrzej
Department of Biochemistry, Faculty of Chemistry, Wrocław University of Technology, Wybrzeze Wyspiańskiego 27, 50-370 Wrocław, Poland.
Biochim Biophys Acta. 2009 Nov;1794(11):1616-24. doi: 10.1016/j.bbapap.2009.07.010. Epub 2009 Jul 25.
Fish otoliths are calcium carbonate biominerals responsible for gravity sensing and the perception of sound. The otoliths formation is controlled by Starmaker (Stm), a protein which belongs to a class of intrinsically disordered proteins. Here, we utilized analytical ultracentrifugation along with Ferguson's analysis of the electrophoretic data to demonstrate that Stm exists in solution as a monomer. The Stm frictional ratio has an unusually high value ranging from 2.6 to 3.1 depending on the method used to analyse the data obtained from analytical ultracentrifugation or gel filtration experiments. These unusually high values of frictional ratio indicate that monomeric Stm has a significantly extended rod-shaped conformation. Calcium ions, which are putative ligands of Stm, induce compaction of the extended conformation of Stm. In particular, increasing the calcium ion concentration from 1 mM to 50 mM lowered the Stokes radius by about 9.5 A. Gel filtration experiments done under denaturing conditions showed only small changes in the dimensions of Stm, which suggests the presence of residual ordered structures. These structures were estimated to be 23% of the Stm structure by detailed analysis of the data obtained by differential scanning microcalorimetry. The elongation of Stm polypeptide chain may facilitate its simultaneous interaction with other components of the composed calcium carbonate crystals which build up otoliths.
鱼类耳石是负责重力感应和声音感知的碳酸钙生物矿物质。耳石的形成受造星蛋白（Stm）控制，该蛋白属于一类内在无序蛋白。在此，我们利用分析超速离心法以及对电泳数据的弗格森分析，证明Stm在溶液中以单体形式存在。Stm的摩擦比具有异常高的值，根据用于分析从分析超速离心或凝胶过滤实验获得的数据的方法不同，其值在2.6至3.1之间。这些异常高的摩擦比值表明单体Stm具有显著延伸的棒状构象。钙离子是Stm的假定配体，可诱导Stm延伸构象的压缩。特别是，将钙离子浓度从1 mM增加到50 mM可使斯托克斯半径降低约9.5 Å。在变性条件下进行的凝胶过滤实验表明，Stm的尺寸只有很小的变化，这表明存在残余的有序结构。通过差示扫描量热法获得的数据的详细分析估计，这些结构占Stm结构的23%。Stm多肽链的伸长可能有助于其与构成耳石的碳酸钙晶体的其他成分同时相互作用。
