Fig. 1. Spectrin subunits in the auditory organ. (A) Spectrins exist as heterotetramers composed of - and β-spectrin subunits [(, β)₂]. The and β subunits present the same structural modular organization, mainly made up of multiple triple-helical repeat units: 21 for subunits, 17 for conventional β subunits and 30 for βV. Other domains, such as the Src-homology domain (SH3) and the EF-hand calmodulin-binding domain in II spectrin, and the N-terminal calponin homology (CH) domains and C-terminal pleckstrin-homology (PH) domain in β spectrins, are indicated. (B) RT-PCR analysis on mouse organ-of-Corti tissue samples at P10 shows the presence of transcripts encoding the II-spectrin and all known β-spectrin subunits, i.e. βI, βII, βIII, βIV and βV. (C-F) Whole mounts of organs of Corti (OC) from P8-P10 mice. Actin filaments (F-actin, red) are labeled with rhodamin-conjugated phalloidin. The II- and βII-spectrin subunits (green; C,D) are detected both in the sensory hair cells and their supporting cells. (E) By contrast, the βV spectrin subunit is detected in the sensory hair cells only. A strong βV spectrin labeling is observed in OHCs (see inset). (F) Serial confocal planes (0.6-µm thick) at different levels of the OHC (see middle panel) show that βV spectrin is restricted to the cortical lattice, along the cell lateral wall. Scale bars: 50 µm (C,E); 5 µm (D,F).