Tracking down protein-protein interactions via a FRET-system using site-specific thiol-labeling

Förster resonance energy transfer is among the most popular tools to follow protein-protein interactions. Although limited to certain cases, site-specific fluorescent labeling of proteins via natural functions by means of chemical manipulations can redeem laborious protein engineering techniques. Herein we report on the synthesis of a heterobifunctional tag and its use in site-specific protein labeling studies aiming at exploring protein-protein interactions. The oxadiazole-methylsulfonyl functionality serves as a thiol specific warhead that enables easy and selective installation of fluorescent labels through a bioorthogonal motif. Mitogen activated protein kinase (MAPK14) and its substrate mitogen activated protein kinase activated kinase (MAPKAP2) or its docking motif, a 22 amino acid-long peptide fragment, were labeled with a donor and an acceptor, respectively. Evolution of strong FRET signals upon protein-protein interactions supported the specific communication between the partners. Using an efficient FRET pair allowed the estimation of dissociation constants for protein-protein and peptide-protein interactions (145 nM and 240 nM, respectively).

Protein disorder prevails under crowded conditions

Crowding caused by the high concentrations of macromolecules in the living cell changes chemical equilibria, thus promoting aggregation and folding reactions of proteins. The possible magnitude of this effect is particularly important with respect to the physiological structure of intrinsically disordered proteins (IDPs), which are devoid of well-defined three-dimensional structures in vitro. To probe this effect, we have studied the structural state of three IDPs, α-casein, MAP2c, and p21(Cip1), in the presence of the crowding agents Dextran and Ficoll 70 at concentrations up to 40%, and also the small-molecule osmolyte, trimethylamine N-oxide (TMAO), at concentrations up to 3.6 M. The structures of IDPs under highly diluted and crowded conditions were compared by a variety of techniques, fluorescence spectroscopy, acrylamide quenching, 1-anilino-8-naphthalenesulfonic acid (ANS) binding, fluorescence correlation spectroscopy (FCS), and far-UV and near-UV circular dichroism (CD) spectroscopy, which allow us to visualize various levels of structural organization within these proteins. We observed that crowding causes limited structural changes, which seem to reflect the functional requirements of these IDPs. α-Casein, a protein of nutrient function in milk, changes least under crowded conditions. On the other hand, MAP2c and, to a lesser degree, p21(Cip1), which carry out their functions by partner binding and accompanying partially induced folding, show signs of local structuring and also some global compaction upon crowded conditions, in particular in the presence of TMAO. The observations are compatible with the possible preformation of binding-competent conformations in these proteins. The magnitude of these changes, however, is far from that of the cooperative folding transitions elicited by crowding in denatured globular proteins; i.e., these IDPs do remain in a state of rapidly interconverting structural ensemble. Altogether, our results underline that structural disorder is the physiological state of these proteins.

The phosphorylation state of threonine-220, a uniquely phosphatase-sensitive protein kinase A site in microtubule-associated protein MAP2c, regulates microtubule binding and stability

Phosphorylation of microtubule-associated protein 2 (MAP2) has a profound effect on microtubule stability and organization. In this work a consensus protein kinase A (PKA) phosphorylation site, T(220), of juvenile MAP2c is characterized. As confirmed by mass spectrometry, this site can be phosphorylated by PKA but shows less than average reactivity among the 3.5 +/- 0.5 phosphate residues incorporated into the protein. In contrast, T(220) is uniquely sensitive to dephosphorylation: three major Ser/Thr protein phosphatases, in the order of efficiency PP2B > PP2A(c) > PP1(c), remove this phosphate group first. MAP2c specifically dephosphorylated at this site binds and stabilizes microtubules stronger than either fully phosphorylated or nonphosphorylated MAP2c. Phosphorylation of this site also affects proteolytic sensitivity of MAP2c, which might represent a further level of control in this system. Thus, the phosphorylation state of T(220) may be a primary determinant of microtubule function.

Autolysis parallels activation of mu-calpain

The kinetics of autolysis and activation of mu-calpain were measured with microtubule-associated protein 2 (MAP2) as a very sensitive substrate. The initial rate of MAP2 hydrolysis was found to be a linear function of the autolysed 76 kDa form of mu-calpain large subunit at both 10 and 300 microM Ca2+, and both straight lines intersected the origin. This finding supports the view that native mu-calpain is an inactive proenzyme and that activation is accompanied by autolysis. The first-order rate constant of autolysis, K1(aut), was determined at different Ca2+ concentrations: the half-maximal value was at pCa2+ = 3.7 (197 microM Ca2+), whereas the maximal value was 1.52 s-1, at 30 degrees C. The Ca(2+)-induced activation process was then monitored by using our novel, continuous fluorimetric assay with labelled MAP2 as substrate. The first-order rate constant of activation, k1(act), was derived as the reciprocal of the lag phase ('transit time') at the initial part of the progress curve: half-maximum was at pCa2+ = 3.8 (158 microM Ca2+) and the maximum value was 2.15 s-1. The good agreement between the kinetic parameters of mu-calpain autolysis and activation is remarkable. We claim that this is the first kinetically correct determination of the rate constant of autolysis of mu-calpain. Pre-activated mu-calpain has a Ca2+ requirement that is almost three orders of magnitude smaller [half-maximal activation at pCa2+ = 6.22 (0.6 microM Ca2+)]. We cannot exclude the possibility that the activation process involves other mechanistic steps, e.g. the rapid dissociation of the mu-calpain heterodimer, but we state that in our conditions in vitro autolysis and activation run in close parallel.

Reticular fibers in the stroma of the thymus

There were investigated thymic biopsies taken from 14 children deceased of intercurrent diseases. After routine paraffin procedures, slides were stained with hematoxylin-eosin, Gordon-Sweet, Wilder and Laidlaw methods. Reticular fibers are described in the stroma and parenchyma of the thymus. Their number and thickness are higher in connective septa, perivascular sheaths, at the cortico-medullary junction and around Hassall's bodies. Their involvement in a basal membrane like structure located between epithelio-reticular cells and thymocytes is discussed.

Mutual protection of microtubule-associated protein 2 (MAP2) and cyclic AMP-dependent protein kinase II against mu-calpain

Phosphorylation by adenosine-3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA), but not by Ca(++)-calmodulin-dependent protein kinase II (CaMK II), was shown earlier to protect microtubule-associated protein 2 (MAP2) from cleavage by m-calpain (Johnson and Foley: J Neurosci Res 34: 642-647, 1993). We reinvestigated this phenomenon with the physiologically more relevant mu-calpain and found a qualitatively similar but quantitatively different picture. We further demonstrate that 1) protection is biphasically dependent on the degree of phosphorylation; 2) Ca(++)-phospholipid-dependent protein kinase (PKC) has about the same effect as PKA; 3) the effects of kinases A and C are not additive; and 4) stripping of native MAP2 from its phosphate content (17.8 +/- 2.3 mol/mol) enhances calpainolysis 3.6-fold. A reciprocal effect between kinase A and MAP2 was found: the RII, but not the RI, regulatory subunit of kinase A, which was shown to bind specifically to MAP2, is protected by MAP2 from mu-calpain attack. It is suggested that the specific anchoring of kinase A-II on MAP2 may serve not only kinase targeting in the dendrites, but also protection from calpainolytic degradation.

An ultrasensitive, continuous fluorometric assay for calpain activity

A rapid, continuous assay for calcium-activated neutral protease activity is described. This assay is based on monitoring the elevation in fluorescence intensity that occurs upon calpainolytic digestion of dichlorotriazinylamino-fluorescein-labeled microtubule-associated protein 2. Tedious separation of peptide products from the protein substrate in this rapid assay is unnecessary, which thus offers two remarkable advantages over conventional caseinolytic assay procedures: (i) it raises sensitivity of detection by about three orders of magnitude, allowing the quantitative determination of calpain in the high picogram range in 10 min; and (ii) it permits a continuous detection of activity, which may prove invaluable in enzyme-mechanism studies that require pre-steady-state measurements. Other features and advantages of the assay, along with its limitations, are discussed in detail.

Ultrastructure of the adipose tissue matrix in children with malnutrition

Bioptic fragments of adipose white tissue taken from trochanterian area from children of 2-22 months old were ultrastructurally investigated. Children were of both sexes, 5 normal and 22 with clinical diagnosis of malnutrition. There were studied many interadipocyte spaces signalling out in cases with malnutrition modifications of different components, some of them related with the degree of malnutrition. There were noted: disorganisation and disappearance of basal membranes of capillaries and glycolema; modifications of endothelial cells with lesions of the capillary wall and free degraded red blood cells; disorganization of the ground substance in small areas or sometimes extended to all matrix of the space; collagen fibres reduced in number and size, and in two cases the presence of collagen fibrils with severe lesions, realeasing an electrondense material, fibrinoid-like; matrix infiltration, in some cases with lipids. In only one interadipocyte space a synaptic button was noted in contact with capillary. In malnutrition lesions of cellular elements of the white adipose tissue the following were observed: adipocytes, fibroblasts, fibrocytes, endothelial cells, mast cells--which in their turn are responsible for modifications of macromolecular structures of the extracellular matrix--glycosaminoglycans, proteoglycans, components of which biosyntheses are cell-dependent.

Trichomonas vaginalis--identification by rapid BPT-Drăgan staining

The blue-polychrome-tannin rapid staining (BPT-Drăgan, 1968) applied on fresh and dried vaginal smears makes possible the identification of different forms of Trichomonas vaginalis (TV) parasite. The nucleus is coloured in blue-violet and the cytoplasm in various degrees of blue-green, violet or bichromatical, with blue and violet zones at the cell periphery. In the cytoplasm, the staining allows the identification of many granules and vacuoles. All the stainable substrates have an acid character and represent morphohistochemical aspects which reflect the parasite metabolism in different functional and evolutive stages. Our results allow the recommendation of the method in the screening actions of the TV identifications.