Fig. 6: Possible scenarios of MYO7A-driven cargo transport in stereocilia.

a Schematic of stereocilia and MET machinery. MET channels are localized to the distal ends of stereocilia and physically connected to the upper tip-link density (UTLD) of adjacent longer stereocilia. b Major interacting partners of MYO7A in stereocilia. SANS and harmonin bridge interactions with other partners. SANS binds to the first MyTH4-FERM domain of MYO7A in an anti-parallel orientation¹²⁵. The PST domain of harmonin b can bind to F-actin²⁶. The single α-helix (SAH) of MYO7A has weak dimerization activity^44,85. SANS, PCDH15 and CDH23 can dimerize with each other^18,19,75 and may keep multiple MYO7A molecules in proximity. Oligomerization has been reported for harmonin⁷⁴. PCDH15 and CDH23 have one transmembrane motif (TM)^18,19. The SH3 domain of MYO7A can interact with the intracellular region (IC) of PCDH15 (CD2 isoform shown)²⁸. c MYO7A activation in stereocilia predicted from the behaviors of MYO7A-RK/AA and MYO7A-ΔSH3-ΔM/F2. From its backfolded autoinhibitory conformation⁷⁰, MYO7A could be dimerized by SANS and harmonin as proposed by a previous X-ray crystallography study⁸⁶ (blue panel). MYO7A may also move processively when a PCDH15 dimer brings two MYO7A molecules into proximity (green panel). Considering the low-frequency processive movement of MYO7A-ΔSH3-ΔM/F2, there may be transitional forms (red panel). d–f Possible MYO7A-driven tip-link assembly. The scaffold-first scenario (d) assumes that MYO7A (with SANS) settles at the future UTLD region via the PST domain of harmonin b and recruits tip-link components. However, MYO7A and SANS lack mechanisms to recognize the future UTLD location. The travel-and-connect scenario (e) assumes that either CDH23 or PCDH15 travels toward stereocilia tips. The other side may simply diffuse. Harmonin isoforms may switch from harmonin a or c (lacking the PST domain) to harmonin b (with the PST domain), given that MYO7A coupled with harmonin b fragments does not traffic efficiently (see Fig. 5). PCDH15 may form temporary links, as indicated by BAPTA-mediated remodeling experiments⁸⁷. The walking-links scenario (f) supported by a previous cryo-electron microscopy study assumes that pre-assembled tip links are transported toward stereocilia tips⁸⁸.