Adaptation through translocation

Phosphorylation of the kinase domain regulates autophosphorylation of myosin IIIA and its translocation in microvilli

Motor activity of myosin III is regulated by autophosphorylation. To investigate the role of the kinase activity on the transporter function of myosin IIIA (Myo3A), we identified the phosphorylation sites of kinase domain (KD), which is responsible for the regulation of kinase activity and thus motor function. Using mass spectrometry, we identified six phosphorylation sites in the KD, which are highly conserved among class III myosins and Ste20-related misshapen (Msn) kinases. Two predominant sites, Thr¹⁸⁴ and Thr¹⁸⁸, in KD are important for phosphorylation of the KD as well as the motor domain, which regulates the affinity for actin. In the Caco2 cells, the full-length human Myo3A (hMyo3AFull) markedly enlarged the microvilli, although it did not show discrete localization within the microvilli. On the other hand, hMyo3AFull(T184A) and hMyo3AFull(T188A) both showed clear localization at the microvilli tips. Our results suggest that Myo3A induces large actin bundle formation to form microvilli, and phosphorylation of KD at Thr¹⁸⁴ and Thr¹⁸⁸ is critical for the kinase activity of Myo3A, and regulation of Myo3A translocation to the tip of microvilli. Retinal extracts potently dephosphorylate both KD and motor domain without IQ motifs (MDIQo), which was inhibited by okadaic acid (OA) with nanomolar range and by tautomycetin (TMC) with micromolar range. The results suggest that Myo3A phosphatase is protein phosphatase type 2A (PP2A). Supporting this result, recombinant PP2Ac potently dephosphorylates both KD and MDIQo. We propose that the phosphorylation–dephosphorylation mechanism plays an essential role in mediating the transport and actin bundle formation and stability functions of hMyo3A.

Interaction of glyceraldehyde-3-phosphate dehydrogenase in the light-induced rod alpha-transducin translocation

The light-dependent subcellular translocation of rod alpha-transducin (GNAT-1, or rod Talpha) has been well documented. In dark-adapted animals, rod Talpha (rTalpha) is predominantly located in the rod outer segment (ROS) and translocates into the rod inner segment (RIS) upon exposure to the light. Neither the molecular participants nor the mechanism(s) involved in this protein trafficking are known. We hypothesized that other proteins must interact with rTalpha to affect the translocations. Using the MBP-rTalpha fusion pulldown assay, the yeast two-hybrid assay and the co-immunoprecipitation assay, we identified glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and rTalpha as interacting proteins. Immunoprecipitation also showed beta-actin associates with rTalpha in the dark but not in the light. To further investigate the involvement of GAPDH in light-induced rod Talpha translocation, GAPDH mRNA was knocked down in vivo by transient expression of siRNAs in rat photoreceptor cells. Under completely dark- and light-adapted conditions, the translocation of rTalpha was not significantly different within the 'GAPDH knock-down photoreceptor cells' compared to the non-transfected control cells. However, under partial dark-adaptation, rTalpha translocated more slowly in the 'GAPDH knock-down cells' supporting the conclusion that GAPDH is involved in rTalpha translocation from the RIS to the ROS during dark adaptation.

Systems-level analysis and evolution of the phototransduction network in Drosophila

Networks of interacting genes are responsible for generating life's complexity and for mediating how organisms respond to their environment. Thus, a basic understanding of genetic variation in gene networks in natural populations is important for elucidating how changes at the genetic level map to higher levels of biological organization. Here, using the well-characterized phototransduction network in Drosophila, we analyze variation in gene expression within and between two closely related species, Drosophila melanogaster and Drosophila simulans, under different environmental conditions. Gene expression levels in the pathway are largely conserved between these two sibling species. For most genes in the network, differences in level of gene expression between species are correlated with degree of polymorphism within species. However, one gene encoding the light-induced ion channel TRPL (transient receptor potential-like) shows an excess of expression divergence relative to polymorphism, suggesting a possible role for natural selection in shaping this expression difference between species. Finally, this difference in TRPL expression likely has significant functional consequences, because it is known that a high level of rhabdomeral TRPL leads to increased sensitivity to dim background light and an increased response to a wider range of light intensities. These results provide a preliminary quantification of variation and divergence of gene expression between species in a known gene network and provide a foundation for a system-level understanding of functional and evolutionary change.

Photoreceptor vitality in organotypic cultures of mature vertebrate retinas validated by light-dependent molecular movements

Vertebrate photoreceptor cells are polarized neurons highly specialized for light absorption and visual signal transduction. Photoreceptor cells consist of the light sensitive outer segment and the biosynthetic active inner segment linked by a slender connecting cilium. The function of mature photoreceptor cells is strictly dependent on this compartmentalization which is maintained in the specialized retinal environment. To keep this fragile morphologic and functional composition for further cell biological studies and treatments we established organotypic retina cultures of mature mice and Xenopus laevis. The organotypic retina cultures of both model organisms are created as co-cultures of the retina and the pigment epithelium, still attached to outer segments of the photoreceptor cells. To demonstrate the suitability of the culture system for physiological analyses we performed apoptotic cell death analyses and verified photoreceptor viability. Furthermore, light-dependent bidirectional movements of arrestin and transducin in photoreceptors in vivo and in the retinal cultures were indistinguishable indicating normal photoreceptor cell-biologic function in organotypic cultures. Our established culture systems allow the analysis of mature photoreceptor cells and their accessibility to treatments, characteristic for common cell culture. Furthermore, this culturing technique also provides an appropriate system for gene delivery to retinal cells and will serve to simulate gene therapeutic approaches prior to difficult and time-consuming in vivo experiments.

Photosensory transduction in unicellular eukaryotes: A comparison between related ciliates Blepharisma japonicum and Stentor coeruleus and photoreceptor cells of higher organisms

Blepharisma japonicum and Stentor coeruleus are related ciliates, conspicuous by their photosensitivity. They are capable of avoiding illuminated areas in the surrounding medium, gathering exclusively in most shaded places (photodispersal). Such behaviour results mainly from motile photophobic response occurring in ciliates. This light-avoiding response is observed during a relatively rapid increase in illumination intensity (light stimulus) and consists of cessation of cell movement, a period of backward movement (ciliary reversal), followed by a forward swimming, usually in a new direction. The photosensitivity of ciliates is ascribed to their photoreceptor system, composed of pigment granules, containing the endogenous photoreceptor -- blepharismin in Blepharisma japonicum, and stentorin in Stentor coeruleus. A light stimulus, applied to both ciliates activates specific stimulus transduction processes leading to the electrical changes at the plasma membrane, correlated with a ciliary reversal during photophobic response. These data indicate that both ciliates Blepharisma japonicum and Stentor coeruleus, the lower eukaryotes, are capable of transducing the perceived light stimuli in a manner taking place in some photoreceptor cells of higher eukaryotes. Similarities and differences concerning particular stages of light transduction in eukaryotes at different evolutional levels are discussed in this article.

Farnesylation of retinal transducin underlies its translocation during light adaptation

G proteins are posttranslationally modified by isoprenylation: either farnesylation or geranylgeranylation. The gamma subunit of retinal transducin (Talpha/Tbetagamma) is selectively farnesylated, and the farnesylation is required for light signaling mediated by transducin in rod cells. However, whether and how this selective isoprenylation regulates cellular functions remain poorly understood. Here we report that knockin mice expressing geranylgeranylated Tgamma showed normal rod responses to dim flashes under dark-adapted conditions but exhibited impaired properties in light adaptation. Of note, geranylgeranylation of Tgamma suppressed light-induced transition of Tbetagamma from membrane to cytosol, and also attenuated its light-dependent translocation from the outer segment to the inner region, an event contributing to retinal light adaptation. These results indicate that, while the farnesylation of transducin is interchangeable with the geranylgeranylation in terms of the light signaling, the selective farnesylation is important for visual sensitivity regulation by providing sufficient but not excessive membrane anchoring of Tbetagamma.

Differential expression and interaction with the visual G-protein transducin of centrin isoforms in mammalian photoreceptor cells

Photoisomerization of rhodopsin activates a heterotrimeric G-protein cascade leading to closure of cGMP-gated channels and hyperpolarization of photoreceptor cells. Massive translocation of the visual G-protein transducin, Gt, between subcellular compartments contributes to long term adaptation of photoreceptor cells. Ca(2+)-triggered assembly of a centrin-transducin complex in the connecting cilium of photoreceptor cells may regulate these transducin translocations. Here we demonstrate expression of all four known, closely related centrin isoforms in the mammalian retina. Interaction assays revealed binding potential of the four centrin isoforms to Gtbetagamma heterodimers. High affinity binding to Gtbetagamma and subcellular localization of the centrin isoforms Cen1 and Cen2 in the connecting cilium indicated that these isoforms contribute to the centrin-transducin complex and potentially participate in the regulation of transducin translocation through the photoreceptor cilium. Binding of Cen2 and Cen4 to Gbetagamma of non-visual G-proteins may additionally regulate G-proteins involved in centrosome and basal body functions.

Sharing time on the fly

The investigation of circadian clock function in Drosophila has progressed from the identification of clock genes to the analysis of timing mechanisms in the cells and tissues where these genes are expressed. As the biological context for investigating circadian clock systems is expanded, new features of molecular timing mechanisms are becoming apparent. Examples come first from studies on peripheral clocks, which perform local, tissue-specific functions as well as global functions that relate to the control of individual behavior, and second from the evaluation of social influences on circadian rhythms. The identification of inter-organismal components of the circadian system in Drosophila suggests new perspectives as the progression continues from the systems level to the social level and onwards to the level of ecosystems.

Light-driven translocation of the protein phosphatase 2A complex regulates light/dark dephosphorylation of phosducin and rhodopsin

In steps of protein purification of bovine retinal protein phosphatase 2A (PP2A), phosducin dephosphorylation activity peaks coelute with a PP2A enzyme complex, shown by peptide sequence analysis to contain a B' subunit, B56 epsilon. Other PP2A complexes with a slightly larger (56.5 kDa) B' subunit (sequenced to be B56 alpha) or with the B alpha regulatory subunit have no phosducin dephosphorylation activity. Upon exposure to light, a significant increase in the immunoreactive protein level of the A, C, and B56 epsilon PP2A subunits is observed in the cytosolic fraction of mouse retina, the phosducin dephosphorylation of which occurs rapidly. During dark exposure, these subunits translocate to the membrane fraction where rhodopsin is slowly dephosphorylated. This PP2A redistribution occurs in less than 1.5 min and is dependent upon light and not upon an intrinsic circadian rhythm. Forty times more of the A subunit (approximately 20 ng/mouse retina) and 9 times more of the C subunit (approximately 4 ng/mouse retina) than of the B56 epsilon subunit (approximately 0.45 ng/mouse retina) redistribute, which suggests that the predominant form of the PP2A enzyme complex on the membrane in the dark is a dimer, consisting of only A and C subunits. We observe that the dimer favors phosphorylated opsin as a substrate, while the trimer, particularly the enzyme complex with the B56 epsilon subunit, greatly prefers phosphorylated phosducin, with an activity several hundred times those of other substrates that were tested. This light-driven PP2A translocation provides a potential mechanism for efficient dephosphorylation of two critical photoreceptor transduction proteins, cytosolic phosducin and membrane-bound rhodopsin, by the same enzyme.