Maturation of major Drosophila rhodopsin, ninaE, requires chromophore 3-hydroxyretinal.

Opsin expression is extremely suppressed by carotenoid deprivation in Drosophila. Carotenoid replacement in deprived flies promotes the recovery of visual pigment with an increase in opsin, as well as the chromophore 11-cis-3-hydroxyretinal. Here, we show that opsin mRNA and opsin peptide in an intermediate step of posttranslational processing were present in carotenoid-deprived flies. By supplementing chromophore to photoreceptor cells, intermediate opsin was made mature. During this process, opsin peptide underwent multiple modifications involving glycosylation. Based on these results, we present a novel mechanism of protein regulatory expression; that is, chromophore posttranslationally controls the expression of apoprotein by promoting its maturation.

A20 targets caspase-8 and FADD to protect HTLV-I-infected cells.

Adult T-cell leukemia (ATL) arises from a human T-cell leukemia virus type I (HTLV-I)-infected cell and has few therapeutic options. Here, we have uncovered a previously unrecognized role for a ubiquitin-editing enzyme A20 in the survival of HTLV-I-infected cells. Unlike in lymphomas of the B-cell lineage, A20 is abundantly expressed in primary ATL cells without notable mutations. Depletion of A20 in HTLV-I-infected cells resulted in caspase activation, cell death induction and impaired tumorigenicity in mouse xenograft models. Mechanistically, A20 stably interacts with caspase-8 and Fas-associated via death domain (FADD) in HTLV-I-infected cells. Mutational studies revealed that A20 supports the growth of HTLV-I-infected cells independent of its catalytic functions and that the zinc-finger domains are required for the interaction with and regulation of caspases. These results indicate a pivotal role for A20 in the survival of HTLV-I-infected cells and implicate A20 as a potential therapeutic target in ATL.

Formation of hyposorhodopsin in frog retina.

Hypsorhodopsin was formed in frog retina by irradiation at liquid helium temperature and converted into bathorhodopsin above about 29K.

Accessibility of the iodopsin chromophore.

Iodopsin can replace its chromophore (11-cis retinal) by added 9-cis retinal, resulting in the formation of isoiodopsin. NaBH4 bleaches iodopsin in the dark. In a relatively low concentration of digitonin, the scotopsin (the protein moiety of chicken rhodopsin) removes 11-cis retinal from iodopsin in the dark. These facts suggest that the linkage of the chromophore to opsin in the iodopsin molecule (presumably a Schiff-base linkage) is accessible to these reagents, which is different from the situation in rhodopsin.

Two forms of intermediates of frog rhodopsin in rod outer segments.

Using frog rod outer segments, we measured changes of the absorption spectrum during the conversion of rhodopsin to a photosteady-state mixture composed of rhodopsin, isorhodopsin and bathorhodopsin by irradiation with blue light (440 nm) at -190 degrees C and during the reversion of bathorhodopsin to a mixture of rhodopsin and isorhodopsin by irradiation with red light (718 nm) at -190 degrees C. The reaction kinetics was expressed by one exponential in the former case and by two exponentials in the latter. These results suggest that rhodopsin is composed of a single molecular species, while bathorhodopsin is composed of two kinds of molecular species designated as batho1-rhodopsin and batho2-rhodopsin. On warming the two forms of bathorhodopsin, each bathorhodopsin converted to its own lumirhodopsin, metarhodopsin I and finally a free all-trans-retinal plus opsin. The absorption spectra of the two forms of bathorhodopsin, lumirhodopsin and metarhodopsin I were measured at -190 degrees C. We infer that a rhodopsin molecule in the excited state relaxes to either batho1-rhodopsin or batho2-rhodopsin, and then converts to its own intermediates through one of the two parallel pathways.

Circular dichroism of squid rhodopsin and its intermediates.

Circular dichroism (CD) and absorption spectra of squid (Todarodes pacificus) rhodopsin, isorhodopsin and the intermediates was measured at low temperatures. Squid rhodopsin has positive CD bands at wavelengths corresponding the alpha- and beta-absorption bands at liquid nitrogen temperature (CD maxima: 485 nm at alpha-band and 348 nm at beta-band) as well as at room temperature (CD maxima: 474 nm at alpha-band and 347 nm at beta-band). The rotational strength of the alpha-band has a molecular ellipticity about twice that of cattle rhodopsin. The CD spectrum of bathorhodopsin displays a negative peak at 532 nm, the rotational strength of which has an absolute value slightly larger than that of rhodopsin. The reversal in sign at alpha-band of the CD spectrum may indicate that the isomerization of retinal chromophore from twisted 11-cis form to twisted 11-trans form has occurred in the process of conversion from rhodopsin to bathorhodopsin. Lumirhodopsin has a small negative CD band at 490 nm, the maximum of which lies at 25 nm shorter wavelengths than the absorption maximum (515 nm), and a large positive CD band near 290 nm, which is not observed in rhodopsin and the other intermediates. This band may de derived from a conformational change of the opsin. In the process of changing from lumirhodopsin to LM-rhodopsin, The CD bands at visible and near ultraviolet regions disappear. Both alkaline and acid metarhodopsins have no CD bands at visible and near ultraviolet regions.

Circular dichroism of cattle rhodopsin and bathorhodopsin at liquid nitrogen temperatures.

The photoevent in vision has been considered to be the conversion of rhodopsin to bathorhodopsin, which is caused by photoisomerization of the chromphoric retinal. Recently some objections were raised to this hypothesis. The reliability of the hypothesis was verified by measurement of circular dichroism of bathorhodopsin. The measurement of circular dichroism of rhodopsin extract (containing 66% or 75% of glycerol) at liquid nitrogen temperatures (-195 degrees C) by a conventional spectropolarimeter induced an extraordinary large signal, owing to linear dichroism originated from conversion of rhodopsin to bathorhodopsin by the measuring light. The similar linear dichrolism can be induced by irradiation of rhodopsin extract at -195 degrees C with polarized light or natural light. At photosteady state the linear dichroism disappeared. Circular dichroism spectrum of cattle rhodopsin displayed two positive peaks ([theta]max = 80 800 degrees at 335 nm, and [theta]max = 42 600 degrees at 500 nm) at -195 degrees C, whereas, bathorhodopsin displayed a positive peak ([theta]max = 43 100 degrees at 334 nm) and a negative peak ([theta]max = 163 000 degrees at 540 nm). The change of the positive sign to negative one at alpha-band of circular dichroism spectrum supports the hypothesis that the conversion of rhodopsin is due to rotation of the chromophoric retinal about C-11--12 double bond ('photoisomerization model').

Molecular cloning of a mouse scavenger receptor with C-type lectin (SRCL)(1), a novel member of the scavenger receptor family.

The cDNA clone encoding a mouse scavenger receptor with C-type lectin (SRCL), a novel member of the scavenger receptor family, has been isolated from a mouse embryonic cDNA library. The predicted cDNA sequence contains a 2226 bp open reading frame encoding a coiled-coil, collagen-like, C-type lectin/carbohydrate recognition domain with an overall sequence identity of 92% to human SRCL. In contrast to human, mouse SRCL mRNA was expressed ubiquitously in various adult tissues including the liver and spleen, in which human SRCL mRNA was under detection limits. Mouse SRCL mRNA was expressed in the macrophage cell line J774A.1 cells at a high level and in the embryo as early as E9.

Isolation and characterization of a thermolabile beta-2 macroglycoprotein ('thermolabile substance' or 'Hakata antigen') detected by precipitating (auto) antibody in sera of patients with systemic lupus erythematosus.

A novel thermolabile beta-2 macroglycoprotein ('thermolabile substance' (TLS) or 'Hakata antigen' (HA], which was detected by the precipitating (auto) antibodies of patients with systemic lupus erythematosus, was isolated and characterized. The purification procedure entailed the following steps: isoelectric precipitation in the range between pH 5.2-6.1, hydroxyapatite absorption chromatography, 35% saturated ammonium sulfate precipitation, Sephadex G-200 gel filtration, Pevikon block electrophoresis, lentil lectin affinity chromatography and immobilized rabbit anti-human whole serum IgG column chromatography. Utilizing these procedures, 0.1 mg of HA was purified from 3 1 of pooled human serum. The molecular mass of HA was determined as 650 kDa by Sepharose 4B gel filtration. On SDS-PAGE analysis, HA showed a single band at 35 kDa under reduced conditions and numerous ladder bands between 35 kDa to more than 300 kDa under nonreduced conditions. On analytical ultracentrifugation, HA gave a molecular mass of 520 kDa with a single meniscus and a sedimentation constant of 12.0. The amino acid and carbohydrate analysis of reduced and S-pyridylethylated HA revealed that it contained five residues of hydroxyproline and an N-linked type sugar chain.

Molecular cloning of two types of cDNA encoding subunit RC6-I of rat proteasomes.

A new subunit, named RC6-I, of the rat 20 S proteasome was purified and the partial amino acid sequences of several peptide fragments obtained by digestion with lysyl-endopeptidase were determined by Edman degradation. Amplification of cDNAs encoding RC6-I by the polymerase chain reaction (PCR) technique revealed two types of cDNA, tentatively designated as RC6-IL and RC6-IS in order of size. The nucleotide sequences of the two cDNAs are identical except that RC6-IL contains an insertion of 18 nucleotides in the coding region compared with RC6-IS. The polypeptide predicted from the open reading frame of RC6-IS cDNA consists of 248 amino acid residues with a calculated molecular weight of 27,783. These values are consistent with those obtained by protein chemical analyses. Computer-assisted homology analysis showed that RC6-I belongs to the alpha-type subfamily of the proteasome gene family, which shows similarity to the alpha-subunit of the archaebacterium Thermoplasma acidophilum proteasome, and that the 18 nucleotide insert, encoding six amino acid residues, VVASVS, appears to be unique to RC6-IL, because this motif has not been conserved in any other alpha-type subunit. By reverse transcription (RT)-PCR analysis, the mRNAs for both RC6-IL and RC6-IS were found in all the rat tissues examined. These results suggest that proteasomes are present as a heterogeneous population, possibly for acquisition of diversity of functions.

Photochemical reaction of 7,8-dihydrorhodopsin at low temperatures.

The photoreaction of 9-cis-7,8-dihydrorhodopsin was examined at liquid nitrogen temperatures (-180 degrees C) in order to elucidate the photochemical events in visual pigments. This rhodopsin analog was prepared by incubating 9-cis-7,8-dihydroretinal with bovine opsin in the dark. 9-cis-7,8-Dihydrorhodopsin (lambda max = 427 nm) was cooled to -180 degrees C, and then irradiated at -180 degrees C with a 390 nm light, resulting in formation of its bathochromic product (lambda max = 465 nm). This result indicates that the presence of four double-bonds adjacent to the Schiff base nitrogen is sufficient to allow formation of a bathochromic product. Thus, the mechanism of formation of bathorhodopsin (in bovine rhodopsin system) may be considered as some change of the interaction between the conjugated double-bond system from C-9 to the Schiff base nitrogen and its surrounding charges in opsin, caused by rotation of 11-12 double-bond.

Two types of sugar-binding protein in the labellum of the fly. Putative taste receptor molecules for sweetness.

Flies have taste cells specifically sensitive to sweetness. It has been suggested that the cells possess two types of receptor sites covering the receptive field of sweetness. By affinity electrophoresis with the site-specific inhibitory polysaccharides, two types of sugar-binding protein were isolated from the labellar extract of the blowfly. These proteins showed consistent sugar-binding specificities and affinities with the two types of receptor sites for sweetness, respectively. The dissociation constant of the protein-sugar complex varies 100-400 mM and the molecular weight of one type of the protein is 27,000, while that of the other is 31,000 or 32,000. Both proteins were water insoluble and were also detected in the isolated chemosensilla. Thus they are probably located on the taste receptor membrane, and the proteins are likely to act as the taste receptor molecules for sweetness in the fly.

Structural characterization of the molten globule and native states of apomyoglobin by solution X-ray scattering.

Compactness and shape are two of the critical properties that describe the degree of protein folding. Solution X-ray scattering is an effective technique for measuring these properties quantitatively. Structural characteristics of various conformational states of horse myoglobin were studied in terms of size and shape by solution X-ray scattering. The radius of gyration for native holomyoglobin was 17.5 A, while that of the apomyoglobin native state was 19.7 A. Corresponding to the increase in the radius of gyration, the largest dimension of the molecule also increased from 47.5 A to 62.5 A. Both states are globular in shape. The scattering profiles in the high angle region suggest that the apomyoglobin native state has a distinct tertiary structure, and that packing of alpha-helices in the apomyoglobin native state would be looser than that of holomyoglobin. These observations indicate that the native state of apomyoglobin is expanded from that of holomyoglobin, and that the conformations of the two are not identical. The radii of gyration for the acid-unfolded state and the denaturant-unfolded state were 30 A and 35 A, respectively. Both unfolded states have chain-like conformations without any tertiary structures. The radius of gyration and the largest dimension of the molten globule stabilized by trichloroacetate were 23.1 A and 72.5 A, respectively. The molten globule is expanded from the native state although it is globular, and is much more compact than the unfolded state. The bimodal distance distribution function and scattering profile at high-angle region suggest that the structure of the apomyoglobin molten globule contains a core comprising a cluster of multiple alpha-helices and flaring tail(s), which would be a common structural property of the compact denatured state appearing during the folding process. The compactness of each conformational state is highly correlated with the extent of formation of the alpha-helix.

Energy coupling in an ion pump. The reprotonation switch of bacteriorhodopsin.

The active site of an ion pump must communicate alternately with the two opposite membrane surfaces. In the light-driven proton pump, bacteriorhodopsin, the retinal Schiff base is first the proton donor to D85 (with access to the extracellular side), and then it becomes the acceptor of the proton of D96 (with access to the cytoplasmic side). This "reprotonation switch" has been associated with a protein conformation change observed during the photocycle. When D85 is replaced with asparagine, the pKa value of the Schiff base is lowered from above 13 to about 9. We determined the direction of the loss or gain of the Schiff base proton in unphotolyzed and in photoexcited D85N, and the D85N/D96N and D85N/D96A double mutants, in order to understand the intrinsic and the induced connectivities of the Schiff base to the two membrane surfaces. The influence of D96 mutations on proton exchange and on acceleration of proton shuttling to the surface by azide indicated that in either case the access of the Schiff base on D85N mutants is to the cytoplasmic side. In the wild-type protein (but with the pKa of the Schiff base lowered by 13-trifluoromethyl retinal substitution) the results suggested that the Schiff base can communicate also with the extracellular side. Raising the pH without illumination of D85N so as to deprotonate the Schiff base caused the same, or nearly the same, change of X-ray scattering as observed when the Schiff base deprotonates during the wild-type photocycle. The results link the charge state of the active site to the global protein conformation and to the connectivity of the Schiff base proton to the membrane surfaces. Their relationship suggests that the conformation of the unphotolyzed wild-type protein is stabilized by coulombic interaction of the Schiff base with its counter-ion. A proton is translocated across the membrane after light-induced transfer of the Schiff base proton to D85, because the protein assumes an alternative conformation that separates the donor from the acceptor and opens new conduction pathways between the active site and the two membrane surfaces.

Structural characterization of the molten globule of alpha-lactalbumin by solution X-ray scattering.

A compact denatured state is often observed under a mild denaturation condition for various proteins. A typical example is the alpha-lactalbumin molten globule. Although the molecular compactness and shape are the essential properties for defining the molten globule, there have been ambiguities of these properties for the molten globule of alpha-lactalbumin. Using solution X-ray scattering, we have examined the structural properties of two types of molten globule of alpha-lactalbumin, the apo-protein at neutral pH and the acid molten globule. The radius of gyration for the native holo-protein was 15.7 A, but the two different molten globules both had a radius of gyration of 17.2 A. The maximum dimension of the molecule was also increased from 50 A for the native state to 60 A for the molten globule. These values clearly indicate that the molten globule is not as compact as the native state. The increment in the radius of gyration was less than 10% for the alpha-lactalbumin molten globule, compared with up to 30% for the molten globules of other globular proteins. Intramolecular disulfide bonds restrict the molecular expansion of the molten globule. The distance distribution function of the alpha-lactalbumin molten globule is composed of a single peak suggesting a globular shape, which is simply swollen from the native state. The scattering profile in the high Q region of the molten globule indicates the presence of a significant amount of tertiary fold. Based on the structural properties obtained by solution X-ray scattering, general and conceptual structural images for the molten globules of various proteins are described and compared with the individual, detailed structural model obtained by nuclear magnetic resonance.

Two types of arrestins expressed in medaka rod photoreceptors.

Similar to visual arrestins of other vertebrates, two subtypes of medaka visual arrestins, KfhArr-R1 and KfhArr-C, are selectively expressed in rods and cones, respectively [Hisatomi et al. (1997) FEBS Lett. 411, 12-18]. We isolated a cDNA encoding the third arrestin, KfhArr-R2, from a medaka retinal cDNA library. Phylogenetic analysis of arrestin sequences suggests that KfhArr-R2 is classified into the rod arrestin subtype. In situ hybridization indicated that KfhArr-R2 mRNA is localized in most of the rod photoreceptors, suggesting that both KfhArr-R1 and -R2 are co-expressed in rods. Antisera against KfhArr-R2 recognized outer segments, but anti-KfhArr-R1 antisera reacted with cell bodies and synaptic termini in light-adapted rods. It seems likely that KfhArr-R1 and -R2 play different roles in rod photoreceptors.

Crystallographic characterization by X-ray diffraction of the M-intermediate from the photo-cycle of bacteriorhodopsin at room temperature.

The structure of the M-intermediate appearing in the photo-cycle of bacteriorhodopsin was studied with X-ray diffraction techniques at room temperature. The lifetime of the M-intermediate was prolonged by treatment with an arginine solution at alkaline pH (Nakasako et al., FEBS Lett. 254, 211-214). The diffraction profile of membranes which had accumulated the M-intermediate had small but significant differences in the intensities of Bragg reflections and the lattice constant in comparison with that of membranes having trans-bacteriorhodopsin. Diffraction intensities were carefully evaluated and the structural changes during the formation of the intermediate were evaluated with difference Fourier analysis. We could find structural changes around helices G and B.

Pinopsin expressed in the retinal photoreceptors of a diurnal gecko.

Retinal cDNAs encoding the putative opsins, dg3 and dg4, were isolated from a diurnal gecko, Phelsuma madagascariensis longinsulae. dg3 mRNA is localized in about 20% of the thin members of type C double cones, and likely encodes an opsin of the ultraviolet-sensitive pigment. Surprisingly, dg4 is very similar to chicken pinopsin, a pineal-specific photoreceptive molecule. An anti-dg4 antiserum recognized a small population of photoreceptor outer segments in the retina and a large number of pinealocytes. Our results suggest that P. m. longinsulae expresses pinopsin in its retina, which usually plays a role as a photoreceptive molecule in the pineal organ.

Arrestins expressed in killifish photoreceptor cells.

Two kinds of cDNA fragments (KfhArr-R and KfhArr-C) encoding the putative arrestins of killifish, Oryzias latipes, were isolated. The distributions of these transcripts were investigated by in situ hybridization, and it was demonstrated that KfhArr-R and KfhArr-C are expressed in, respectively, rod and all four types of cone cells. The deduced amino acid sequences of KfhArr-R and KfhArr-C are closely related to human S-antigen (rod arrestin) and X-arrestin (cone arrestin), respectively. Phylogenetic analysis of arrestin sequences suggests that vertebrate visual arrestins form a single cluster distinct from other arrestins and diverged to form rod and cone subtypes before the divergence between teleosts and tetrapods. It is speculated that the divergence pattern of vertebrate visual arrestins may prove to be reflected in the divergence of the proteins participating in the respective phototransduction cascades.