Structural plasticity of the HHD2 domain of whirlin

Abstract

Whirlin is a protein essential to sensory neurons. Its defects are responsible for nonsyndromic deafness or for the Usher syndrome, a condition associating congenital deafness and progressive blindness. This large multidomain scaffolding protein is expressed in three isoforms with different functions and localizations in stereocilia bundles of hearing hair cells or in the connecting cilia of photoreceptor cells. The HHD2 domain of whirlin is the only domain shared by all isoforms, but its function remains unknown. In this article, we report its crystal structure in two distinct conformations, a monomeric five-helix bundle, similar to the known structure of other HHD domains, and a three-helix bundle organized as a swapped dimer. Most of the hydrophobic contacts and electrostatic interactions that maintain the globular monomeric form are conserved at the protomer interface of the dimer. NMR experiments revealed that the five-helix conformation is predominant in solution, but exhibits increased dynamics on one face encompassing the hinge loops. Using NMR and SAXS, we also show that HHD2 does not interact with its preceding domains. Our findings suggest that structural plasticity might play a role in the function of the HHD2 domain.

Deciphering the Unexpected Binding Capacity of the Third PDZ Domain of Whirlin to Various Cochlear Hair Cell Partners

“…Previously we reported the HHD2 domain does not interact with the N-terminal PDZ1 and PDZ2 domains and binding partners of the HHD2 are currently unknown [29]. Overall, our results demonstrated that the PDZ3 domain is independent from the other folded domain HHD2, in the larger molecular context of whirlin as it is linked to the rest of the molecule by flexible sequence.…”

“…As illustrated by the IUPRED plot (https://iupred2a.elte.hu; [28]) (Fig. 1), this large linker region is predicted to be intrinsically disordered and tethers the two well-defined folded structures of HHD2 domain and PDZ3 domain [29].…”

Temporal and spatial assembly of inner ear hair cell ankle link condensate through phase separation

“…There are two HHDs on WHRN: HHD1 and HHD2. The structure of the WHRN HHD2 has been reported (denoted as MD in the present study) 34 . However, little is known about HHD1 (denoted as NTD in the present study).…”

Structure of the Harmonin PDZ2 and coiled‐coil domains in a complex with CDHR2 tail and its implications

“…Both whirlin and PDZD7 contain an HHD domain but they perform different functions. The HHD of whirlin was reported to undergo a conformational change to form a swapped dimer, 56 whereas the HHD of PDZD7 was shown to be attached to the membrane via direct binding to lipid 57 . The different structures and functions of these regions in the three proteins suggested that they evolved independently to accommodate their distinct physiological roles.…”