Search PubMed⌕ Search

Biomedical subjects

J Schwemer

Publications and source records attributed to J Schwemer.

16 recordsLinked to original sources
PubMed1996

Spin-labeling analysis of structure and dynamics in octopus rhodopsin.

The location of cysteines accessible in octopus rhodopsin were characterized by a spin-labeling technique. Two cysteines were found to bind a methanthiosulfonate spin label. One of the spin labels is attached to helix V with the side chain located within the membrane, most probably close to the polar head group region. The second spin label was found to be attached to cysteine 345 in the C terminus. Light-induced reversible electron paramagnetic resonance spectral changes were observed for the spin label attached at position 345. It is concluded that conformational changes occur during the rhodopsin to metarhodopsin transition in the vicinity of the C-terminus position 345.

Animals↗
PubMed1993

Frequenin--a novel calcium-binding protein that modulates synaptic efficacy in the Drosophila nervous system.

The T(X;Y)V7 rearrangement in Drosophila has originally been recognized as a Shaker-like mutant because of its behavioral and electrophysiological phenotype. The gene whose expression is altered by the V7 rearrangement has been characterized. It encodes a novel Ca(2+)-binding protein named frequenin, which is related to recoverin and visinin. In vitro, the frequenin protein functions like recoverin as a Ca(2+)-sensitive guanylyl cyclase activator. Anti-frequenin antibodies stain the central and peripheral nervous system in Drosophila embryos and in larval and adult tissue sections. Frequenin appears to be particularly enriched in synapses, such as the motor nerve endings at neuromuscular junctions. Neuromuscular junctions of transgenic flies, which overexpress frequenin upon heat shock, exhibit an extraordinarily enhanced, frequency-dependent facilitation of neurotransmitter release, with properties identical to those observed in V7 junctions. We propose that frequenin represents a new element for the Ca(2+)-dependent modulation of synaptic efficacy.

Amino Acid Sequence↗
PubMed1990

Rhodopsin reconstitution in bleached rod outer segment membranes in the presence of a retinal-binding protein from the honeybee.

The physiological role of a retinal-binding protein from honeybee is investigated. This protein, upon previous loading with all-trans retinal and subsequent irradiation with monochromatic light of wavelength 490 nm, is able to promote the reconstitution of rhodopsin when added to a suspension of opsin membranes from bleached bovine rod outer segments. In this respect this retinal-binding protein could have a role very similar to that postulated for the well-known cephalopod retinochrome, that serves to catalyze the formation in the presence of light of 11-cis retinal in photo-receptor cells and to provide it for the reconstitution of rhodopsin during the visual cycle.

Animals↗
PubMed1987

Resonance raman spectroscopy of an ultraviolet-sensitive insect rhodopsin.

We present the first visual pigment resonance Raman spectra from the UV-sensitive eyes of an insect, Ascalaphus macaronius (owlfly). This pigment contains 11-cis-retinal as the chromophore. Raman data have been obtained for the acid metarhodopsin at 10 degrees C in both H2O and D2O. The C = N stretching mode at 1660 cm-1 in H2O shifts to 1631 cm-1 upon deuteriation of the sample, clearly showing a protonated Schiff base linkage between the chromophore and the protein. The structure-sensitive fingerprint region shows similarities to the all-trans-protonated Schiff base of model retinal chromophores, as well as to the octopus acid metarhodopsin and bovine metarhodopsin I. Although spectra measured at -100 degrees C with 406.7-nm excitation, to enhance scattering from rhodopsin (lambda max 345 nm), contain a significant contribution from a small amount of contaminants [cytochrome(s) and/or accessory pigment] in the sample, the C = N stretch at 1664 cm-1 suggests a protonated Schiff base linkage between the chromophore and the protein in rhodopsin as well. For comparison, this mode also appears at approximately 1660 cm-1 in both the vertebrate (bovine) and the invertebrate (octopus) rhodopsins. These data are particularly interesting since the absorption maximum of 345 nm for rhodopsin might be expected to originate from an unprotonated Schiff base linkage. That the Schiff base linkage in the owlfly rhodopsin, like in bovine and in octopus, is protonated suggests that a charged chromophore is essential to visual transduction.

Animals↗
PubMed1984

Morphological correlates of visual pigment turnover in photoreceptors of the fly, Calliphora erythrocephala.

The content of visual pigment in one eye of Calliphora 'chalky' was measured spectrophotometrically and related to the ultrastructure of the other eye and the density of membrane particles as revealed by the freeze-fracture technique. Electron microscopy of thin-sectioned material showed that the manifestations of synthesis and breakdown of photoreceptor membrane were most prominent in flies kept in blue light, in comparison to flies kept in green light or darkness, in which only a moderate breakdown of membrane was evident. By subjecting flies to different light regimes it was found that the density of membrane particles was related to the content of visual pigment, but not on a one-to-one basis. In particular, the particle density in flies with a low (less than 10%) rhodopsin content, produced by raising flies on a vitamin A-deficient diet or by exposure to green light, was still about 35% of that of normal flies. Taken together the results indicate that all rhodopsin molecules are particles but that some particles represent another protein that most probably serves to maintain the structural integrity of the photoreceptor membrane. Furthermore, membrane synthesis can take place in the absence of rhodopsin synthesis.

Animals↗
PubMed1983

Biogenesis of blowfly photoreceptor membranes is regulated by 11-cis-retinal.

Biogenesis of photoreceptor membranes has been investigated by analyzing the rhodopsin and opsin content of microvillar photoreceptor membranes after injecting retinal isomers and radioactive amino acids into the compound eyes of carotenoid-deficient blowflies. The amount of rhodopsin in the membranes was measured photometrically in extracts of isolated rhabdoms. The opsin content of the membranes and the level of radioactive labelling of opsin were measured after separating the membrane proteins by dodecyl sulphate/polyacrylamide gel electrophoresis. In the photoreceptor membrane of carotenoid-deficient flies the rhodopsin and opsin content is less than 4% of that in normal flies where opsin may constitute about 65% of the total membrane protein. In carotenoid-deficient flies the incorporation of rhodopsin into photoreceptor membranes is triggered by 11-cis-retinal but not all-trans retinal. After injection of 11-cis-retinal the opsin content of the photoreceptor membranes increases in parallel with the rhodopsin content. Radioactive labelling of opsin reveals that 11-cis-retinal triggers an incorporation of newly synthesized opsin into photoreceptor membranes whereas all-trans retinal does not induce the assembly of opsin into photoreceptor membranes. Light-dependent incorporation of radioactive labelled opsin into membranes with a high rhodopsin content shows that not only the visual pigment chromophore but also the opsin undergoes a light-dependent turnover. The findings raise the possibility that opsin synthesis in blowfly photoreceptors and consequently the assembly of photoreceptor membranes is regulated by 11-cis-retinal.

Animals↗
PubMed1982

Characteristics of retinal-binding proteins from the honeybee retina.

Spectrophotometric studies were performed on two water soluble retinal-binding proteins isolated from honeybee retina. Both pigments, B and C, absorb maximally at about 440 nm. Pigment B is bleached by light to a photoproduct with lambda max at about 370 nm. This pigment reacts with hydroxylamine in the dark to form a product with an absorbance maximum at 360 nm, whereas with cyanoborohydride it reacts only in the light forming a product with lambda max at about 330 nm. Irradiation of pigment C also leads to the formation of a photoproduct with lambda max at about 370 nm but, in contrast to that of pigment B, it reconverts to its 440 nm-form during the following dark period. The results obtained by changing the pH of the extracts support the hypothesis that all-trans retinal binds to each protein via a Schiff base linkage (pK of 8.4). The data are discussed with relation to the physiological role pigment B could play in the visual cycle of honeybees.

Animals↗
PubMed1982

Visual pigment and visual receptor cells in fetal and adult sheep.

The visual pigments, and the structure of the visual cells, were investigated by spectrophotometry and by light and electron microscopy in fetal and adult sheep. The rhodopsin system in adult sheep closely resembles that of cattle. The absorbance maxima of rhodopsin, lumirhodopsin, and metarhodopsin I are at 498, 490, and 480 nm, respectively. The estimated molar absorbance coefficient at the wavelength of maximum absorption of rhodopsin is 40,000M-1 . cm-1. Rhodopsin was detected from a fetal age of 85 days (term at 145 days). Partial bleaching of extracts from fetal eyes (95, 105, and 115 days) did not demonstrate cone pigments, although cones were present in fair numbers at a fetal age of about 105 days. The time course of rhodopsin formation between 85 and 140 days resembles a growth curve. The amount of rhodopsin shortly before birth (140 days) is about 0.6 times that in the adult. The number and dimensions of rod outer segments as well as packing of the discs were studied structurally and related to the rhodopsin content. A fairly good correlation was found at the earliest stages (95 and 105 days gestation age), when rhodopsin concentration was very low and rod outer segments were few and small, as well as at the latest stage (140 days). At 115 days the rhodopsin content observed by spectrophotometry was less than that indicated by the outer segment volume, probably mainly due to the outer segment discs and possibly to the rhodopsin molecules being less tightly packed than at 140 days.

Animals↗
PubMed1980

Spectral transmission of the ocular media of the pegion (Columba livia).

Spectrophotometric and microspectrophotometric measures of relative spectral transmission were obtained from the cornea, lens, and vitreous body and from the whole eye of young and adult pigeons. Transmission was above 90% throughout the visible spectrum and was maintained at or above 50% into the near-UV at 310 nm. No age-related changes in transmission were found. The results concur with the pigeon's behavioral UV-detection abilities.

Age Factors↗
PubMed1979

Vitamin A deficiency reduces the concentration of visual pigment protein within blowfly photoreceptor membranes.

Visual pigment extracts prepared from rhabdomeric membranes of vitamin A deficient blowflies contain a 5-10 times lower concentration of rhodopsin than extracts from flies which were raised on a vitamin A rich diet. Spectrophotometry showed that digitonin-solubilized rhodopsin from blowfly photoreceptors R1-6 has an absorbance maximum at about 490 nm, but no unusually enhanced beta-band in the ultraviolet. The extracts did not contain detectable concentrations of other visual pigments nor was there any evidence for the presence of photostable vitamin A derivatives. Sodium dodecyl sulfate polyacrylamide gel electrophoresis demonstrated that the concentration of opsin in the rhabdomeric membrane is significantly reduced in vitamin A deficient flies compared to normal flies. The results indicate that the synthesis of opsin or its incorporation into the photoreceptor membrane is regulated by the chromophore concentration in the receptor cell. Furthermore, our findings open up the possibility that differences in the spectral absorption and excitability of photoreceptors from normal and vitamin A deficient flies result from the differing opsin content of the rhabdomeres.

Animals↗