The giant spectrin βV couples the molecular motors to phototransduction and Usher syndrome type I proteins along their trafficking route

Abstract

Mutations in the myosin VIIa gene cause Usher syndrome type IB (USH1B), characterized by deaf-blindness. A delay of opsin trafficking has been observed in the retinal photoreceptor cells of myosin VIIa-deficient mice. We identified spectrin βV, the mammalian β-heavy spectrin, as a myosin VIIa- and rhodopsin-interacting partner in photoreceptor cells. Spectrin βV displays a polarized distribution from the Golgi apparatus to the base of the outer segment, which, unlike that of other β spectrins, matches the trafficking route of opsin and other phototransduction proteins. Formation of spectrin βV-rhodopsin complex could be detected in the differentiating photoreceptors as soon as their outer segment emerges. A failure of the spectrin βV-mediated coupling between myosin VIIa and opsin molecules thus probably accounts for the opsin transport delay in myosin VIIa-deficient mice. We showed that spectrin βV also associates with two USH1 proteins, sans (USH1G) and harmonin (USH1C). Spectrins are supposed to function as heteromers of α and β subunits, but fluorescence resonance energy transfer and in vitro binding experiments indicated that spectrin βV can also form homodimers, which likely supports its αII-independent βV functions. Finally, consistent with its distribution along the connecting cilia axonemes, spectrin βV binds to several subunits of the microtubule-based motor proteins, kinesin II and the dynein complex. We therefore suggest that spectrin βV homomers couple some USH1 proteins, opsin and other phototransduction proteins to both actin- and microtubule-based motors, thereby contributing to their transport towards the photoreceptor outer disks.

Spectrin βV adaptive mutations and changes in subcellular location correlate with emergence of hair cell electromotility in mammalians

“…Together, these results suggest that in mammalian vestibular hair cells, spectrin βV, through its interaction with Rab8, but without the assistance of spectrin αII, is involved in the directional translocation of tethered vesicular membranes to the plasma membrane. This conclusion is further supported by the previously reported interaction of spectrin βV with the dynein and kinesin complexes and with myosin VIIa, motor proteins that move cargos along microtubules and along actin filaments, respectively (30). Therefore, the involvement of nonconventional β spectrins in cargo transport seems to be evolutionarily conserved, despite differences in the subcellular location of these proteins between nonmammalian and mammalian cells.…”

“…For immunofluorescence and immunoblot analyses, the reported antibodies were used as described in ref. 30. Protein-protein and protein-lipid interaction assays were carried out as described in ref.…”

Spectrin Tetramer Formation Is Not Required for Viable Development in Drosophila

“…It has been suggested that the ␣I/␤I-spectrin might form oligomeric structures without the H2H interaction (3) and it was recently shown that human ␤ H -spectrin can self-associate (19). Such results raise the possibility that non-conventional networks may exist, and clearly their role would not be tested by the current work.…”

“…Alternative oligomerization mechanisms based on lateral interactions between ␣-spectrin and ␤-spectrin have been proposed (17) but remain equivocal (18). Recently, it was shown that human ␤ H -spectrin can self-associate (19), raising the possibility that F-actin cross-linking could be mediated by this ␤ isoform in the absence of ␣-spectrin. Clearly there is potential for diversity although none has yet been demonstrated.…”

Spectrin: Structure, function and disease

“…Spectrin V homodimers are associated with USH1 proteins, opsin, and other phototransduction proteins that are transported toward the photoreceptor outer disks [70]. Also, spectrin serves as a biomarker for detecting several brain related injuries.…”