Cross-linking of bovine rhodopsin with sulfosuccinimidyl 4-(N maleimidomethyl)cyclohexane-1-carboxylate affects its functionality.

Rhodopsin is the photoreceptor protein involved in visual excitation in retinal rods. The functionality of bovine rhodopsin was determined following treatment with sulfosuccinimidyl 4-(N maleimidomethyl)cyclohexane-1-carboxylate (sulfo-SMCC), a bifunctional reagent capable of forming covalent cross-links between suitable placed lysines and cysteines. Denaturing polyacrylamide gel electrophoresis showed that rhodopsin incubated with sulfo-SMCC generated intermolecular dimers, trimers, and higher oligomers, although most of the sulfo-SMCC-treated protein remained as a monomer. Minor alterations on the absorption spectrum of light-activated sulfo-SMCC-treated rhodopsin were observed. However, only ∼2% stimulation of the guanine nucleotide binding activity of transducin was measured in the presence of sulfo-SMCC-cross-linked photolyzed rhodopsin. Moreover, rhodopsin kinase was not able of phosphorylating sulfo-SMCC-cross-linked rhodopsin after illumination. Rhodopsin was purified in the presence of either 0.1% or 1% n-dodecyl ß-d-maltoside, to obtain dimeric and monomeric forms of the protein, respectively. Interestingly, no generation of the regular F1 and F2 thermolytic fragments was perceived with sulfo-SMCC-cross-linked rhodopsin either in the dimeric or monomeric state, implying the formation of intramolecular connections in the protein that might thwart the light-induced conformational changes required for interaction with transducin and rhodopsin kinase. Structural analysis of the rhodopsin three-dimensional structure suggested that the following lysine and cysteine pairs: Lys66/Lys67 and Cys316, Cys140 and Lys141, Cys140 and Lys248, Lys311 and Cys316, and/or Cys316 and Lys325 are potential candidates to generate intramolecular cross-links in the protein. Yet, the lack of fragmentation of sulfo-SMCC-treated Rho with thermolysin is consistent with the formation of cross-linking bridges between Lys66/Lys67 and Cys316, and/or Cys140 and Lys248.

A variant of arrestin-1 binds rod outer segment membranes in a light-independent manner.

A 50-kDa-polypeptide band peripherally bound to retinal rod outer segment (ROS) membranes was purified by anion-exchange chromatography. When the 50-kDa protein was compared with purified arrestin-1, it was observed that: (1) both proteins comigrated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and were recognized by either anti-50-kDa protein polyclonal antibodies or anti-arrestin-1 monoclonal antibodies; (2) protein fragments and peptide fingerprint maps obtained following limited and complete proteolysis with specific proteases were very similar for both molecules; and (3) several chromatographically-purified tryptic peptides from the 50-kDa protein possessed the same amino acid composition as tryptic peptides deduced from the reported arrestin-1 primary structure. Consequently, arrestin-1 and the purified 50-kDa protein must correspond to variants of the same molecule. However, in contrast to arrestin-1 that associated to the ROS membranes only in the presence of light and ATP, the 50-kDa protein interacted with the ROS membranes in a light-independent manner, either in the presence or absence of ATP. These results clearly established that phosphorylated and illuminated rhodopsin is not the membrane anchor for this variant of arrestin-1.

Purification of a Src family tyrosine protein kinase from bovine retinas.

Since tyrosine phosphorylation appears to play important functions in photoreceptor cells, we searched here for retinal nonreceptor tyrosine kinases of the Src family. We demonstrated that Src family tyrosine kinases were present in the cytosolic fraction of extracted bovine retinas. A Src family tyrosine kinase with an apparent molecular mass of about 62 kDa was purified to homogeneity from the soluble fraction of dark-adapted bovine retinas after three consecutive purification steps: ω-aminooctyl-agarose hydrophobic chromatography, Cibacron blue 3GA-agarose pseudo-affinity chromatography, and α-casein-agarose affinity chromatography. The purified protein was subjected to N-terminal amino acid sequencing and the sequence Gly-Ile-Ile-Lys-Ser-Glu-Glu was obtained, which displayed homology with the first seven residues of the Src family tyrosine kinase c-Yes from Bos taurus (Gly-Cys-Ile-Lys-Ser-Lys-Glu). Although the cytosolic fraction from dark-adapted retinas contained tyrosine kinases of the Src family capable of phosphorylating the α-subunit of transducin, which is the heterotrimeric G protein involved in phototransduction, the purified tyrosine kinase was not capable of using transducin as a substrate. The cellular role of this retinal Src family member remains to be found.

85-kDa protein of Trypanosoma cruzi purified by affinity chromatography used in the multiple antigen binding assay (MABA) for the diagnosis of T. cruzi infection in a Venezuelan rural community.

No ideal test exists for Chagas' disease, and better diagnostic strategies are needed. We determined the diagnostic utility of an 85-kDa Trypanosoma cruzi protein in a multiple antigen binding assay (MABA). A standardized MABA test based on concentrated trypomastigote excretory-secretory antigen (TESA) and an 85-kDa purified protein showed 100% sensitivity and specificity. In field conditions, 6/66 individuals tested in a region not thought to be endemic (Rio Brito) were identified as seropositive for T. cruzi infection with our MABA test. In parallel, an enzyme-linked immunosorbent assay based on fixed epimastigotes detected 7/66 positives, which were independently confirmed. These data suggest that the 85-kDa and TESA proteins could be used in the MABA format as a complementary tool for the diagnosis of latent Chagas' disease. High anti-T. cruzi antibody detection rates, poor knowledge of Chagas' disease and its vector, and the demonstration of infected vectors in the study community all suggest a significant risk of reemergence of T. cruzi infection in this region of Venezuela.

Light or tyrosine phosphorylation recruits retinal rod outer segment proteins to lipid rafts.

Lipid rafts are localized liquid-ordered regions of the plasma membrane that contain high levels of cholesterol and glycosphingolipids, and are resistant to extraction with nonionic detergents. Retinal photoreceptor cells contain detergent-resistant membrane microdomains (DRM), which were isolated here from bovine rod outer segments (ROS) under dark and light conditions. Rhodopsin (R) was present in both DRM and detergent soluble fractions (DSF), and detergent-insoluble ROS rafts were enriched in caveolin 1 (Cav-1) and c-Src. In the dark, arrestin and its 44-kDa truncated form (p44) were present mainly in DSF; however, p44 was translocated to DRM under illumination. Similarly, transducin (T) was mainly present in DSF in the dark, but it was recruited toward the DRM fraction following photolysis. DRM were also prepared in the absence or presence of Mg-ATP, guanosine 5'-3-O-(thio)triphosphate (GTPγS), or both. Although GTPγS released T into DSF in the light, GTPγS-activated T was retained in DRM when Mg2+ and ATP were added. Moreover, T was always tyrosine-phosphorylated under light conditions, which suggested that T phosphorylation prevents its GTPγS-induced release from DRM. In addition, treatment with the tyrosine kinase inhibitor genistein prevented the segregation of T to the rafts. In contrast, no localization difference was seen in the presence of Mg-ATP for Cav-1, c-Src, R and both forms of arrestin. Interestingly, immunoprecipitation assays followed by Western blot analyses under light conditions showed the formation of multimeric complexes containing R, T, c-Src, p44 and Cav-1 in DRM, where T and c-Src were tyrosine-phosphorylated.

Correlation of transducin photoaffinity labeling with the specific formation of intermolecular disulfide linkages in its α-subunit.

Transducin (T) is a heterotrimer of Tα, Tß, and Tγ subunits. In the presence of light-activated rhodopsin, 8-azidoguanosine triphosphate (8-N3GTP) was covalently incorporated into T in a UV-light photodependent manner, with a low stoichiometry of 0.02 mol of 8-N3GTP per mol of T. Although Tα was preferentially labeled by 8-N3GTP, Tß and Tγ were also modified. Photolabeling of T was specifically inhibited by GDP and GTP, but not by ß,γ-imido-guanosine 5'-triphosphate (GMP-PNP), indicating that 8-N3GTP was modifying the GDP binding site of the holoenzyme. This was consistent with the observation that the photoaffinity probe was completely hydrolyzed to 8-N3GDP by T activated by illuminated rhodopsin. The formation of intermolecular disulfide associations in T was also determined because photolabeling of T was performed under non-reducing conditions. We established that Cys-347 of Tα was the major residue involved in the formation of disulfide-linked T oligomers. Other cysteines of Tα, such as Cys-321, also participated in the formation of disulfide bonds, revealing a complex pattern of intermolecular disulfide cross-links that led to the polymerization of T. The spontaneous generation of these cystines in Tα inhibited the light-dependent GTPase and GMP-PNP binding activities of T. A model was constructed illustrating that when two heterotrimers dimerize through the formation of disulfide bridges between the Cys-347 of their Tα subunits, the guanine ring of the 8-N3GDP bound to one T molecule might approach to the Tßγ-complex of the other heterotrimer. This model provides an explanation for the additional photolabeling of Tß and Tγ by 8-N3GTP.

Chemical modification of transducin with dansyl chloride hinders its binding to light-activated rhodopsin.

Transducin (T), the heterotrimeric guanine nucleotide binding protein in rod outer segments, serves as an intermediary between the receptor protein, rhodopsin, and the effector protein, cGMP phosphodiesterase. Labeling of T with dansyl chloride (DnsCl) inhibited its light-dependent guanine nucleotide binding activity. Conversely, DnsCl had no effect on the functionality of rhodopsin. Approximately 2-3 mol of DnsCl were incorporated per mole of T. Since fluoroaluminate was capable of activating DnsCl-modified T, this lysine-specific labeling compound did not affect the guanine nucleotide-binding pocket of T. However, the labeling of T with DnsCl hindered its binding to photoexcited rhodopsin, as shown by sedimentation experiments. Additionally, rhodopsin completely protected against the DnsCl inactivation of T. These results demonstrated the existence of functional lysines on T that are located in the proximity of the interaction site with the photoreceptor protein.

La actividad de la tutoría en las carreras de las ciencias médicas / Tutorship in Medical Sciences specialties

RESUMEN La tutoría en la Educación Superior actual, se desarrolla de forma flexible y se acomoda a cada uno de los estudiantes de acuerdo con su personalidad, intereses, conocimientos, capacidades, nivel de dificultades y avances. Es un sistema de educación que a la vez que atiende las características personales del estudiante de manera individualizada, actúa dentro de un sistema de educación colectiva, contribuyendo a la formación integral de los estudiantes. Dentro de esta modalidad académica juega un papel integrador el tutor ya que es el que realiza el trabajo hombre a hombre en aras de lograr un profesional de la salud con una concepción humana capaz de lograr los reto de la sociedad moderna, además es el eje fundamental en la preparación profesional, por lo que debe ejercer un conjunto de influencias educativas e instructivas para que el estudiante logre apropiarse de las habilidades, hábitos y valores que son indispensables en su formación. El objetivo del trabajo consiste en valorar la tutoría en la educación superior haciendo énfasis en la educación médica (AU).

ABSTRACT Tutorship in the current High Education develops in a flexible way, and is adjusted to every one of the students, according to their personality, interests, knowledge, skills, difficulties level and advances. It is an educational system that, while pays attention to the student's personal characteristics in an individual way, behaves inside a system of collective education, rendering contribution to the comprehensive training of the students. In this academic way, the tutor plays an integrative role, because he is the one who carries out the man-to-man work for the sake of training a health professional with a human conception able of affronting the challenges of the modern society. Besides that, he is the main stone in the professional training, and therefore has to exert a whole of educative and training influences to make the student to develop skills, habits and values that are essential in his training. The objective of this paper is to assess tutorship in High Education, making emphasis in medical education (AU).

La actividad de la tutoría en las carreras de las ciencias médicas / Tutorship in Medical Sciences specialties

RESUMEN La tutoría en la Educación Superior actual, se desarrolla de forma flexible y se acomoda a cada uno de los estudiantes de acuerdo con su personalidad, intereses, conocimientos, capacidades, nivel de dificultades y avances. Es un sistema de educación que a la vez que atiende las características personales del estudiante de manera individualizada, actúa dentro de un sistema de educación colectiva, contribuyendo a la formación integral de los estudiantes. Dentro de esta modalidad académica juega un papel integrador el tutor ya que es el que realiza el trabajo hombre a hombre en aras de lograr un profesional de la salud con una concepción humana capaz de lograr los reto de la sociedad moderna, además es el eje fundamental en la preparación profesional, por lo que debe ejercer un conjunto de influencias educativas e instructivas para que el estudiante logre apropiarse de las habilidades, hábitos y valores que son indispensables en su formación. El objetivo del trabajo consiste en valorar la tutoría en la educación superior haciendo énfasis en la educación médica (AU).

ABSTRACT Tutorship in the current High Education develops in a flexible way, and is adjusted to every one of the students, according to their personality, interests, knowledge, skills, difficulties level and advances. It is an educational system that, while pays attention to the student's personal characteristics in an individual way, behaves inside a system of collective education, rendering contribution to the comprehensive training of the students. In this academic way, the tutor plays an integrative role, because he is the one who carries out the man-to-man work for the sake of training a health professional with a human conception able of affronting the challenges of the modern society. Besides that, he is the main stone in the professional training, and therefore has to exert a whole of educative and training influences to make the student to develop skills, habits and values that are essential in his training. The objective of this paper is to assess tutorship in High Education, making emphasis in medical education (AU).

Distribution of dehydration rates generated by maximal Gardos-channel activation in normal and sickle red blood cells.

The Ca(2+)-activated K+ channels of human red blood cells (RBCs) (Gardos channels, hIK1, hSK4) can mediate rapid cell dehydration, of particular relevance to the pathophysiology of sickle cell disease. Previous investigations gave widely discrepant estimates of the number of Gardos channels per RBC, from as few as 1 to 3 to as many as 300, with large cell-to-cell differences, suggesting that RBCs could differ extensively in their susceptibility to dehydration by elevated Ca2+. Here we investigated the distribution of dehydration rates induced by maximal and uniform Ca2+ loads in normal (AA) and sickle (SS) RBCs by measuring the time-dependent changes in osmotic fragility and RBC volume distributions. We found a remarkable conservation of osmotic lysis and volume distribution profiles during Ca(2+)-induced dehydration, indicating overall uniformity of dehydration rates among AA and SS RBCs. In light of these results, alternative interpretations were suggested for the previously proposed low estimates and heterogeneity of channel numbers per cell. The results support the view that stochastic Ca2+ permeabilization rather than Gardos-channel variation is the main determinant selecting which SS cells dehydrate through Gardos channels in each sickling episode.

The hydrodynamic properties of dark- and light-activated states of n-dodecyl beta-D-maltoside-solubilized bovine rhodopsin support the dimeric structure of both conformations.

Rhodopsin (Rho) has been extracted in n-dodecyl beta-D-maltoside (DM) from bovine retinal rod outer segments and purified to homogeneity by affinity chromatography on concanavalin A-Sepharose. Because chemical cross-linking of Rho and photoactivated Rho (Rho*) provided initial evidence for the oligomeric nature of the photoreceptor protein, we carried out a hydrodynamic characterization of the native and activated conformations of detergent-solubilized Rho. The molecular weights of the complexes between dark and photoexcited states of Rho and DM were determined by gel filtration chromatography on Sephacryl S-300, in the presence of 0.1% DM. Subtracting the size of the corresponding detergent micelles resulted in molecular masses of 78 kDa for native Rho and 76 kDa for Rho*. The measured content of 0.97 g of detergent/g of protein resulted in a calculated partial specific volume of 0.765 cm(3)/g for the protein-detergent complex and a molar mass of 64-65 kDa for the protein moiety. The sizes of Rho.DM and Rho*.DM complexes were also evaluated by sedimentation on 10-30% sucrose gradients, in the presence of 0.1% DM, and molecular masses of about 60 kDa were estimated for both the dark- and light-activated states of the photoreceptor protein. The size of Rho was determined to be 65,300 and 69,800 Da, respectively, when the purified Rho.DM complex was either chromatographed on Sephacryl S-300 or ultracentrifuged on sucrose gradients in the absence of DM. All these results were consistent with a dimeric quaternary structure for both conformations of Rho. Additionally, the functional integrity of the purified photoreceptor protein following gel filtration chromatography and ultracentrifugation was demonstrated by three criteria as follows: (i) its characteristic UV-visible absorption spectra, (ii) its capability to photoactivate transducin, and (iii) its ability to serve as a substrate for rhodopsin kinase.

Identification of functionally important acidic residues in transducin by group-specific labeling.

Transducin (T), a GTP-binding protein involved in phototransduction of rod photoreceptor cells, is a heterotrimer arranged as two units, the alpha-subunit (T alpha) and the beta gamma-complex (T beta gamma). The role of the carboxyl groups in T was evaluated by labeling with N,N'-dicyclohexylcarbodiimide (DCCD) and 1-ethyl 3-(3-dimethylaminopropyl) carbodiimide (EDC). Only a minor effect on the binding of beta, gamma-imido guanosine 5'-triphosphate (GMPpNp) to T was observed in the presence of the hydrophobic carbodiimide, DCCD. Similarly, the GMPpNp binding activity of the reconstituted holoenzyme was not significantly affected when T alpha was combined with DCCD-treated T beta gamma. However, the binding of guanine nucleotides to the reconstituted T was approximately 50% inhibited when DCCD-labeled T alpha was incubated with T beta gamma. In contrast, treatment of T with the hydrophilic carbodiimide, EDC, completely impaired its GMPpNp-binding ability. EDC-modified T was incapable of interacting with illuminated rhodopsin, as determined by sedimentation experiments. However, rhodopsin only partially protected against the inactivation of T. Additionally, analyses of trypsin digestion patterns showed that fluoroaluminate was not capable of activating the EDC-labeled T sample. The function of the reconstituted holoenzyme was also disrupted when EDC-modified T alpha was combined with T beta gamma, and when EDC-treated T beta gamma was incubated with T alpha.

Distribution of plasma membrane Ca2+ pump activity in normal human red blood cells.

The plasma membrane calcium pump (PMCA) is the only active Ca2+ transporter in human red blood cells (RBCs). Previous measurements of maximal Ca2+ extrusion rates (Vmax) reported only mean values in the RBC population. Despite early evidence for differences in Ca2+ extrusion capacity among RBCs, the precise Vmax distribution remained unknown. It was important to characterize this distribution to assess the range and modality (uni- or multimodal) of PMCA Vmax variation and the likelihood of RBCs with elevated [Ca2+]i in the circulation participating in physiologic and pathologic processes. We report here the application of a new method to investigate the detailed distribution of PMCA Vmax activity in RBCs. The migrating profile of osmotic lysis curves was used to identify and quantify the fraction of cells that extrude a uniform Ca2+ load at different rates. The results revealed that RBCs from single donors have large variations in PMCA activity that follow a unimodal, broad distribution pattern consistently skewed toward higher Vmax values, suggesting an excess of cells with Vmax higher than the mean value. The method applied may provide a way of evaluating whether the observed variation in PMCA Vmax is related to cell age.

Aproximaciones de bioquímica clásica al estudio de la relación entre la estructura y la función de la rodopsina Y / Approaches to the study of classical biochemistry of the relationship between structure and function of the rhodopsin

La proteína fotorreceptora rodopsina (R) fue extraída de los segmentos externos de los bastoncillos de retinas bovinas con el detergente n-dodecil β-D-maltósido (DM) y purificada a homogeneidad mediante cromatografía de afinidad. El entrecruzamiento químico de la R y de la rodopsina fotoactivada (R*) con los agentes bifuncionales sulfo-succinimidilo 4-(N-maleimidometilo) ciclohexano-1-carboxilato (sulfo-SMCC) o m-maleimidobenzoilo-N-hidroxisuccinimido ester, sugirieron la naturaleza oligomérica de la proteína fotorreceptora. La caracterización de los parámetros hidrodinámicos de la R y la R* en presencia de 0.1% DM, mediante cromatografía de exclusión molecular y sedimentación sobre gradientes de sacarosa, permitió estimar los tamaños de los complejos R:DM y R*: DM. Los resultados concuerdan con una estructura cuaternaria dimérica tanto para la R como para la R*. La R entrecruzada con sulfo-SMCC, en presencia de luz, fue estabilizada en un fotointermediario que absorbió a ~ 470 nm. Experimentos de proteólisis con termolísina sobre los dímeros nativos de R y sobre los monómeros de R generados por medio del uso de altas concentraciones de DM, complementados con estudios de modelaje basados en la estructura cristalina reportada de la proteína, sugirieron que el reactivo sulfo-SMCC generó un entrecruzamiento intramolecular entre la Cys140 y la Lys248 de la R, el cual posiblemente es el responsable de la incapacidad de la proteína de sufrir el cambio conformacional requerido para llegar a su estado fotoactivado.

Identification of functionally important acidic residues in transducin by group-specific labeling

Transducin (T), a GTP-binding protein involved in phototransduction of rod photoreceptor cells, is a heterotrimer arranged as two units, the alpha-subunit (T alpha) and the beta gamma-complex (T beta gamma). The role of the carboxyl groups in T was evaluated by labeling with N,N'-dicyclohexylcarbodiimide (DCCD) and 1-ethyl 3-(3-dimethylaminopropyl) carbodiimide (EDC). Only a minor effect on the binding of beta, gamma-imido guanosine 5'-triphosphate (GMPpNp) to T was observed in the presence of the hydrophobic carbodiimide, DCCD. Similarly, the GMPpNp binding activity of the reconstituted holoenzyme was not significantly affected when T alpha was combined with DCCD-treated T beta gamma. However, the binding of guanine nucleotides to the reconstituted T was approximately 50 per cent inhibited when DCCD-labeled T alpha was incubated with T beta gamma. In contrast, treatment of T with the hydrophilic carbodiimide, EDC, completely impaired its GMPpNp-binding ability. EDC-modified T was incapable of interacting with illuminated rhodopsin, as determined by sedimentation experiments. However, rhodopsin only partially protected against the inactivation of T. Additionally, analyses of trypsin digestion patterns showed that fluoroaluminate was not capable of activating the EDC-labeled T sample. The function of the reconstituted holoenzyme was also disrupted when EDC-modified T alpha was combined with T beta gamma, and when EDC-treated T beta gamma was incubated with T alpha.