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.

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The Drosophila melanogaster ecdysteroid receptor (EcR) is a member of the nuclear hormone receptor superfamily. EcR controls animal development and metamorphosis by activating or repressing the transcription of target genes. There are three EcR isoforms, EcRA, EcRB1, and EcRB2 that exhibit diverse spatial and temporal distributions within various tissues and reveal essential functional differences. These differences can be attributed to the isoform-specific N-terminal domains (NTDs), which differ in length and primary structure. To lay a foundation for understanding of the molecular mechanism underlying functional diversity of the isoforms, we have carried out a comprehensive biochemical and biophysical analysis of purified hexahistidine-tagged EcRA and EcRB1 NTDs (EcRA-NTD and EcRB1-NTD). The results, along with in silico examinations of the primary structures indicate that the EcR NTDs exhibit properties of premolten globule-like intrinsically disordered proteins. Furthermore, we demonstrate for the first time that NTDs of isoforms of a particular nuclear hormone receptor exhibit distinct structural properties. In silico analysis revealed that the EcRA-NTD sequence has a bigger tendency for disorder than the EcRB1-NTD sequence. Accordingly, the circular dichroism experiments demonstrated that EcRA-NTD has lower regular secondary structure content than EcRB1-NTD and the size-exclusion chromatography showed that EcRA-NTD is less compact than EcRB1-NTD. Furthermore, the limited proteolysis analysis revealed that the C-terminal region common to both NTDs is more susceptible to the enzymatic cleavage in EcRA-NTD than in EcRB1-NTD. We postulate that unique conformational states of EcRA-NTD and EcRB1-NTD might act as the starting points for the functional diversity of EcRA and EcRB1 isoforms.

Focal adhesion protein paxillin links integrin and growth factor signaling to actin cytoskeleton. Most of paxillin signaling activity is regulated via leucine-rich LD motifs (LD1-LD5) located at the N-terminus. Here, we demonstrate a method to engineer highly selective synthetic antibodies (sABs) against LD2 and LD4 which are binding sites for focal adhesion kinase (FAK) and other proteins. Phage display selections against peptides were used to generate sABs recognizing each LD motif. In the obtained X-ray crystal structures of the LD-sAB complexes the LD motifs are helical and bind sABs through a hydrophobic side, similarly as in the structures with natural paxillin partners. The sABs are capable of pulling down endogenous paxillin in complex with FAK and can visualize paxillin in focal adhesions in cells. They were also used as selective inhibitors to effectively compete with focal adhesion targeting domain of FAK for the binding to LD2 and LD4. The sABs are tools for investigation of paxillin LD binding “platforms” and are capable of inhibiting paxillin interactions; and thereby useful as potential therapeutics in the future.