The N-terminal cytoplasmic domain of neuregulin 1 type III is intrinsically disordered

Axonally expressed neuregulin 1 (NRG1) type III is a transmembrane protein involved in various neurodevelopmental processes, including myelination and Schwann cell migration. NRG1 type III has one transmembrane domain and a C-terminal extracellular segment, which contains an epidermal growth factor homology domain. Little is known, however, about the intracellular N terminus of NRG1 type III, and the structure-function relationships of this cytoplasmic domain have remained uncharacterized. In the current study, we carried out the first structural and functional studies on the NRG1 type III cytoplasmic domain. Based on sequence analyses, the domain is predicted to be largely disordered, while a strictly conserved region close to the transmembrane segment may contain helical structure and bind metal ions. As shown by synchrotron radiation circular dichroism spectroscopy, the recombinant NRG1 type III cytoplasmic domain was disordered in solution, but it was able to fold partially into a helical structure, especially when both metals and membrane-mimicking compounds were present. NRG1 cytoplasmic tail binding to metals was further confirmed by calorimetry. These results suggest that the juxtamembrane segment of the NRG1 type III cytoplasmic domain may fold onto the membrane surface upon metal binding. Using synchrotron small-angle X-ray scattering, we further proved that the NRG1 cytoplasmic domain is intrinsically disordered, highly elongated, and behaves like a random polymer. Our work provides the first biochemical and biophysical data on the previously unexplored cytoplasmic domain of NRG1 type III, which will help elucidate the detailed structure-function relationships of this domain.