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

Hum Mol Genet. 2013 Sep 15;22(18):3773-88. doi: 10.1093/hmg/ddt228. Epub 2013 May 23.

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adaptor Proteins, Signal Transducing / metabolism
  • Animals
  • Cell Cycle Proteins
  • Cytoskeletal Proteins
  • HeLa Cells
  • Humans
  • Light Signal Transduction
  • Mice
  • Microtubule Proteins / metabolism
  • Myosin VIIa
  • Myosins / metabolism*
  • Photoreceptor Cells, Vertebrate / metabolism*
  • Retina / metabolism
  • Rhodopsin / metabolism
  • Spectrin / genetics*
  • Spectrin / metabolism*
  • Usher Syndromes / metabolism

Substances

  • Adaptor Proteins, Signal Transducing
  • Cell Cycle Proteins
  • Cytoskeletal Proteins
  • MYO7A protein, human
  • Microtubule Proteins
  • Myo7a protein, mouse
  • Myosin VIIa
  • USH1C protein, human
  • Spectrin
  • Rhodopsin
  • Myosins