Phototransduction is a well characterized system for study of G protein coupled receptor (GPCR) signaling. The GPCR rhodopsin couples to the heterotrimeric G protein transducin. Light-stimulated activation of transducin in turn activates phosphodiesterase (PDE), leading to closure to cGMP-gated channels and inhibition of glutamate release. Rod and cone photoreceptors are highly polarized neurons consisting of the outer segment (OS) where phototransduction biochemistry occurs, the inner segment containing mitochondria and other organelles, the nuclear layer, an axon, and a glutamatergic synapse. Upon illumination, activated G protein transducin redistributes from the rod OS (where it is localized in the dark) to the inner compartments of the cell. Interestingly, cone transducin does not translocate in light. Opposite to this, visual arrestin migrates from the inner compartments to the OS, where it binds to rhodopsin. Previous reports from other groups and our lab argue for either an active or passive mechanism for transducin and arrestin redistribution. Our lab has shown that arrestin migration occurs by diffusion which is restricted by molecular sinks (Nair et al, 2005b). The focus of my dissertation was to unravel the molecular mechanism of rod transducin translocation. Specifically, I found energy (ATP) was not required for transducin movement within photoreceptors. Also, I found that the disc membranes of the rod outer segments as well as protein-protein interactions with retinal guanylate cyclase serve to restrict transducin diffusion through the cell. In addition, I used the insights gained from these studies of transducin to re-examine the relationship of other G proteins' subcellular localization and signal transduction. Ultimately, I found that most G proteins do not undergo subunit dissociation under physiological activating conditions.

The signalling cascade of rods and cones use different but related protein components. Rods and cones, emerged in the common ancestor of vertebrates around 500 million years ago around when two whole genome duplications took place, named 1R and 2R. These generated a large number of additional genes that could evolve new or more specialised functions. A third event, 3R, occurred in the ancestor of teleost fish.  This thesis describes extensive phylogenetic and comparative synteny analyses of the opsins, transducin and phosphodiesterase (PDE6) of this cascade by including data from a wide selection of vertebrates. The expression of the zebrafish genes was also investigated. The results show that genes for these proteins duplicated in 1R and 2R as well as some in 3R. Expression analyses of the zebrafish genes revealed additional specialisations for the 3R gene duplicates. The transducin beta subunit genes, gnb1a and gnb1b, show co-localisation in rods but are expressed at different levels. Gnb3a and gnb3b show different expression in the adult retina with low expression of gnb3a and expression of gnb3b in cones of the dorso-medial retina. The transducin gamma subunit genes gngt2a and gngt2b are expressed in the ventral and dorso-medial retina respectively. The both of PDE6 gamma subunit genes, pde6ga and pde6gb are both expressed in rods but pde6ga shows rhythmic changes of expression with low daytime levels. Pde6ha and pde6hb are expressed in cones however pde6ha show high daytime expression. All investigated transducin and PDE6 subunit genes, but gnb1b, were also expressed in the adult pineal complex or at some point during development. These results provide compelling evidence that the 1R and 2R genome duplications facilitated the evolution of rods and cones by generating gene duplicates that could evolve distinct expression and function. This supports existence of colour vision before the origin of vertebrates, elaboration of this in the early vertebrate ancestor, along with origin of the black-and-white dim-light vision of rods. Furthermore, the different expression patterns observed in the zebrafish retina for teleost 3R duplicates demonstrate multiple additional specialisations.

No description available.

Thesis (M.Sc.). / Written for the Dept. of Biochemistry. Title from title page of PDF (viewed 2008/12/10). Includes bibliographical references.

Das Außensegment der Stäbchenzelle ist aus einem Stapel von flachen Membransäckchen, den Diskmembranen, aufgebaut. Entlang dessen existiert ein Cholesterolgradient mit 24 mol% Cholesterol in den basalen Diskmembranen und 5 mol% in den apikalen. Das Außensegment enthält alle Proteine der Signaltransduktion. Der Photorezeptor Rhodopsin ist als integrales Membranprotein in die Diskmembran eingebettet. Das G-Protein Transducin und das Effektorprotein, die Phosphodiesterase (PDE), sind periphere Proteine mit Lipidankern und somit reversibel mit der Membranoberfläche assoziiert. Um den Einfluss des Cholesterolgehaltes der Diskmembranen auf diese drei Proteine zu untersuchen, wurden Diskmembranen mit unterschiedlichem Cholesterolgehalt präpariert (Simulation des Cholesterolgradienten). Die Untersuchungen zur transversalen Verteilung des Cholesterols in der Diskmembran ergaben eine schnelle Transmembranbewegung mit einer Halbwertzeit von weniger als einer Minute bei 35 °C. Desweiteren konnte gezeigt werden, dass es zu kopfgruppenspezifischen Wechselwirkungen von Cholesterol mit dem Phospholipid Phosphatidylcholin kommt. Cholesterol verschiebt das Meta I-Meta II-Gleichgewicht (nach Lichtaktivierung von Rhodopsin) auf die Seite von Meta I (inaktiv). In dieser Arbeit konnte jedoch gezeigt werden, dass durch die Anwesenheit des Transducins das Gleichgewicht vollständig auf die Seite von Meta II (aktiv) verschoben wird, da Transducin spezifisch die Meta-II-Form stabilisiert. Somit kann die verminderte Meta II-Bildung des Rezeptors in Diskmembranen mit hohem Cholesterolgehalt durch Transducin ausgeglichen werden. Lediglich die Geschwindigkeit der Transducinaktivierung ist verlangsamt. Durch den erhöhten Cholesterolgehalt werden die Membraneigenschaften für eine Bindung der beiden peripheren Proteine Transducin und PDE über deren Lipidanker optimiert. Somit kann die Signaltransduktion auch in den basalen Diskmembranen des Stäbchenaußensegmentes stattfinden. / The rod outer segment consists of a stack of flat membrane saccules called disc membranes. Along this stack a cholesterol gradient exists with 24 mol% cholesterol in the basal and only 5 mol% in the apical disc membranes. The outer segment contains all the proteins necessary for signal transduction. The photoreceptor rhodopsin as integral membrane protein is embedded in the disc membrane. The G protein transducin and the effector protein phosphodiesterase (PDE) are soluble proteins with lipid modifications, which are associated reversibly to the membrane surface. Disc membranes with different cholesterol contents were prepared to simulate the cholesterol gradient along the rod outer segment and to investigate the influence of disc membrane cholesterol content of these three proteins. Investigations of the transversal distribution of cholesterol in the disc membrane revealed a fast transmembrane movement with a half life of less than one minute at 35 °C. Further, head group specific interactions between cholesterol and phosphatidylcholine could be shown. The Meta I Meta II equilibrium after light activation of rhodopsin was shifted to the Meta I (inactive) site in membranes with high cholesterol. In this work it was shown that in the presence of transducin this equilibrium is shifted completely to the Meta II (active) site because transducin stabilizes specifically the Meta II form of the receptor. Hence the reduced Meta II formation in disc membranes with high cholesterol could be compensated by transducin. The speed of transducin activation is decelerated. By the increased cholesterol content membrane properties are optimized to the binding of transducin and PDE via their lipid modifications. Thus the signal transduction can take place also in disc membranes with high cholesterol.

Thesis (Ph. D.)--University of Washington, 2002. / Vita. Includes bibliographical references (leaves 74-81).

No description available.

The duplication of genes followed by selection is perhaps the most prominent way in which molecular biological systems gain multiplicity, diversity and functional complexity in evolution. Whole genome duplications (WGDs) therefore have the potential of generating an extraordinary amount of evolutionary innovation. It is now accepted that the vertebrate lineage has gone through two rounds of WGD in its early stages, after the divergence of invertebrate chordates and before the emergence of jawed vertebrates. These basal vertebrate WGDs are called 2R for two rounds of whole genome duplication. An additional WGD called 3R occurred early in the evolution of teleost fishes, before the radiation of this species-rich group. This thesis describes the evolution of several endocrine and neuronal gene families in relation to the vertebrate WGDs, through a comparative genomic approach including both phylogenetic analyses and chromosomal location data across a wide range of vertebrate taxa. These results show that numerous endocrine gene families have expanded in 2R and in several cases also in 3R. These include the gene families of oxytocin and vasopressin receptors (OT/VP-R), somatostatin receptors (SSTR) and insulin-like growth factor binding proteins (IGFBP). For the OT/VP-R and SSTR families, previously undescribed subtypes were identified. The protein hormone family that includes growth hormone (GH), prolactin (PRL) and somatolactin (SL) acquired a new PRL gene in 2R, however the origins of GH, PRL and SL likely predate 2R. The corresponding family of receptors diversified during different time periods through a combination of local duplications and 3R. Neuronal gene families of the visual system have also expanded in 2R and 3R. The results presented here demonstrate that the vertebrate repertoire of visual opsin genes arose in 2R as part of chromosomal blocks that also include the OT/VP-R genes. The gene families including the transducin alpha, beta and gamma subunits also arose in 2R, hinting at the importance of these events in the diversification and specialization of phototransduction cascades for rods and cones. Thus, the whole genome duplications have been important contributors to the evolution of both vision and endocrine regulation in the vertebrates.