The effect of counter ions on the conformation of intrinsically disordered proteins studied by size-exclusion chromatography.

Counter ions are able to change the conformation of intrinsically disordered proteins (IDPs) to a more compact structure via the reduction of electrostatic repulsion. When the extended IDP conformation is transformed into a more ordered one, the value of the Stokes radius should decrease. Size-exclusion chromatography is a simple method for the determination of the Stokes radius, which describes the hydrodynamic properties of protein molecules. In our paper size-exclusion chromatography experiments of Starmaker (a highly acidic IDP), in the presence of various counter ions, are presented as an example of a simple experimental method, which provides valuable information about subtle counter ions-induced conformational changes in IDP.

Intrinsic disorder of Drosophila melanogaster hormone receptor 38 N-terminal domain.

Drosophila hormone receptor 38 (dHR38), an ortholog of the vertebrate NR4A subclass of nuclear receptors, responds to ecdysteroids, which mediate developmental transitions during the Drosophila life cycle. However, this response is independent of the ecdysteroid receptor, and it does not involve binding of ecdysteroids to dHR38. It has been suggested that ecdysteroids may indirectly activate dHR38, perhaps by recruiting specific proteins. There have been recent reports pointing out the decisive role that nuclear receptor N-terminal domains (NTDs) have in protein-protein interactions that are important for regulation of gene expression. It is reasonable to assume that dHR38-NTD may also be involved in some protein-protein interactions that are critical for the ecdysteroid signaling pathway. To facilitate the exploration of the molecular basis of these interactions, we developed and optimized a protocol for the efficient expression and purification of the recombinant dHR38-NTD. Using a diverse array of biochemical and biophysical methods, we carried out the first structural characterization of dHR38-NTD. The results of our study indicate that dHR38-NTD exhibits a characteristic reminiscent of pre-molten globule-like intrinsically disordered proteins existing in a partially unfolded conformation with regions of secondary structures. The dHR38-NTD structure, which apparently comprises some local, ordered, tertiary structure clusters, is pliable and can adopt more ordered conformations in response to changes in environmental conditions. Thus, dHR38-NTD, which exhibits the structural and functional characteristic of a pre-molten globule-like intrinsically disordered protein, could serve as a platform for multiple protein-protein interactions, possibly including interactions with proteins involved in an unusual ecdysteroid signaling pathway.

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 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.

Starmaker exhibits properties of an intrinsically disordered protein.

Fish otoliths composed of calcium carbonate and an organic matrix play a primary role in gravity sensing and the perception of sound. Starmaker (Stm) was the first protein found to be capable of influencing the process of biomineralization of otoliths. Stm dictates the shape, size, and selection of calcium carbonate polymorphs in a concentration-dependent manner. To facilitate exploration of the molecular basis of Stm function, we have developed and optimized a protocol for efficient expression and purification of the homogeneous nontagged Stm. The homogeneous nontagged Stm corresponds to its functional form, which is devoid of a signal peptide. A comprehensive biochemical and biophysical analysis of recombinant Stm, along with in silico examinations, indicate for the first time that Stm exhibits the properties of intrinsically disordered proteins. The functional significance of Stm having intrinsically disordered protein properties and its possible role in controlling the formation of otoliths is discussed.