The rod-shaped conformation of Starmaker

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 Å. 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.