Evidence for Myosin VIIa-Driven Transport of Rhodopsin in the Plasma Membrane of the Photoreceptor-Connecting Cilium

Abstract

Defects in the gene encoding for the unconventional myosin VIIa leads to human Usher syndrome 1B, the most common form of hereditary combined blindness and deafness. To determine cellular function of myosin VIIa, we have investigated the subcellular localization of myosin VIIa in spacial relation relationship to potentially interacting proteins in mammalian photoreceptor cells. Western blot analysis of the axonemal fraction of photoreceptor cells by Western blot show that myosin VIIa and actin, as well as opsin, were present in the ciliary portion of the photoreceptors. Improved immunoelectron microscopy revealed that in mammalian photoreceptor cells, myosin VIIa was localized at the membrane of the connecting cilium linking photoreceptor inner segments with their outer segments. In addition, actin, the functional partner of myosins, actin, was detected at the ciliary membrane providing considerable evidence that myosin VIIa also interacts with actin filaments in the connecting cilium. Applying the highly-sensitive silver-enhanced indirect nanogold™-labeling for immunoelectron microscopy, intense anti-opsin immunoreactivity was recognized in the membrane of the connecting cilium indicating that rhodopsin trafficing to the outer segment occurs through the ciliary membrane. Co-localization of rhodopsin with the myosin VIIa-actin systems at the membrane also provides evidence for mysion VIIa driven ciliary transport of rhodopsin. Absence of myosin VIIa or presence of non-functioning myosin VIIa may lead to defects in the ciliary transport and cause retinitis pigmentosa in Usher syndrome 1B patients.