Discovery and Characterization of Linear Motif Mediated Protein-Protein Complexes

There are myriads of protein-protein complexes that form within the cell. In addition to classical binding events between globular domains, many protein-protein interactions involve short disordered protein regions. The latter contain so-called linear motifs binding specifically to ordered protein domain surfaces. Linear binding motifs are classified based on their consensus sequence, where only a few amino acids are conserved. In this chapter we will review experimental and in silico techniques that can be used for the discovery and characterization of linear motif mediated protein-protein complexes involved in cellular signaling, protein level and gene expression regulation.

Phosphorylation-Assisted Luciferase Complementation Assay Designed to Monitor Kinase Activity and Kinase-Domain-Mediated Protein-Protein Binding

Protein kinases are key regulators of cell signaling and have been important therapeutic targets for three decades. ATP-competitive drugs directly inhibit the activity of kinases but these enzymes work as part of complex protein networks in which protein-protein interactions (often referred to as kinase docking) may govern a more complex activation pattern. Kinase docking is indispensable for many signaling disease-relevant Ser/Thr kinases and it is mediated by a dedicated surface groove on the kinase domain which is distinct from the substrate-binding pocket. Thus, interfering with kinase docking provides an alternative strategy to control kinases. We describe activity sensors developed for p90 ribosomal S6 kinase (RSK) and mitogen-activated protein kinases (MAPKs: ERK, p38, and JNK) whose substrate phosphorylation is known to depend on kinase-docking-groove-mediated protein-protein binding. The in vitro assays were based on fragment complementation of the NanoBit luciferase, which is facilitated upon substrate motif phosphorylation. The new phosphorylation-assisted luciferase complementation (PhALC) sensors are highly selective and the PhALC assay is a useful tool for the quantitative analysis of kinase activity or kinase docking, and even for high-throughput screening of academic compound collections.

A non-catalytic herpesviral protein reconfigures ERK-RSK signaling by targeting kinase docking systems in the host

The Kaposi's sarcoma associated herpesvirus protein ORF45 binds the extracellular signal-regulated kinase (ERK) and the p90 Ribosomal S6 kinase (RSK). ORF45 was shown to be a kinase activator in cells but a kinase inhibitor in vitro, and its effects on the ERK-RSK complex are unknown. Here, we demonstrate that ORF45 binds ERK and RSK using optimized linear binding motifs. The crystal structure of the ORF45-ERK2 complex shows how kinase docking motifs recognize the activated form of ERK. The crystal structure of the ORF45-RSK2 complex reveals an AGC kinase docking system, for which we provide evidence that it is functional in the host. We find that ORF45 manipulates ERK-RSK signaling by favoring the formation of a complex, in which activated kinases are better protected from phosphatases and docking motif-independent RSK substrate phosphorylation is selectively up-regulated. As such, our data suggest that ORF45 interferes with the natural design of kinase docking systems in the host.

Peptide Based Inhibitors of Protein Binding to the Mitogen-Activated Protein Kinase Docking Groove

Mitogen-activated protein kinases (MAPK) are important regulatory units in cells and they take part in the regulation of many cellular functions such as cell division, differentiation or apoptosis. All MAPKs have a shallow docking groove that interacts with linear binding motifs of their substrate proteins and their regulatory proteins such as kinases, phosphatases, scaffolds. Inhibition of these protein–protein interactions may reduce or abolish the activity of the targeted kinase. Based on the wide range of their biological activity, this kind of inhibition can be useful in the treatment of many disorders like tumors, inflammation or undesired cell apoptosis. In this study a linear binding motif from the RHDF1 protein—a 15 amino acids long peptide—was selected for optimization to increase its cellular uptake but retaining its low micromolar binding affinity. First, we synthesized an octaarginine conjugate that showed efficient cellular uptake. Next, we set out to reduce the size of this construct. We were able to decrease the length of the original peptide, and to increase its cellular uptake with specific chemical modifications. These new constructs bound better to ERK2 and p38 kinases than the original peptide and they showed markedly increased cellular uptake. The new octaarginine conjugate and one of the minimized bicyclic derivatives could inhibit the phosphorylation of intracellular ERK or p38. However, the modulation of MAPK phosphorylation levels by these cell-penetrating peptides were complex, despite that in biochemical assays they all inhibited MAPK-substrate binding as well as phosphorylation. The optimized peptides depending on the applied concentration caused an expected decrease, but also some unexpected increase in MAPK phosphorylation patterns in the cell. This possibly reflects the complexity of MAPK docking groove mediated protein–protein interactions including bone fide MAPK clients such activator kinases, deactivating phosphatases or regulatory scaffolds. Thus, our findings with optimized cell-penetrating “inhibitory” peptides highlight the opportunities but also the pitfalls of docking peptide based MAPK activity regulation and call for a better quantitative understanding of MAPK mediated protein–protein interactions in cells.

Co-regulation of the transcription controlling ATF2 phosphoswitch by JNK and p38

Transcription factor phosphorylation at specific sites often activates gene expression, but how environmental cues quantitatively control transcription is not well-understood. Activating protein 1 transcription factors are phosphorylated by mitogen-activated protein kinases (MAPK) in their transactivation domains (TAD) at so-called phosphoswitches, which are a hallmark in response to growth factors, cytokines or stress. We show that the ATF2 TAD is controlled by functionally distinct signaling pathways (JNK and p38) through structurally different MAPK binding sites. Moreover, JNK mediated phosphorylation at an evolutionarily more recent site diminishes p38 binding and made the phosphoswitch differently sensitive to JNK and p38 in vertebrates. Structures of MAPK-TAD complexes and mechanistic modeling of ATF2 TAD phosphorylation in cells suggest that kinase binding motifs and phosphorylation sites line up to maximize MAPK based co-regulation. This study shows how the activity of an ancient transcription controlling phosphoswitch became dependent on the relative flux of upstream signals.

MAP Kinase-Mediated Activation of RSK1 and MK2 Substrate Kinases

Mitogen-activated protein kinases (MAPKs) control essential eukaryotic signaling pathways. While much has been learned about MAPK activation, much less is known about substrate recruitment and specificity. MAPK substrates may be other kinases that are crucial to promote a further diversification of the signaling outcomes. Here, we used a variety of molecular and cellular tools to investigate the recruitment of two substrate kinases, RSK1 and MK2, to three MAPKs (ERK2, p38α, and ERK5). Unexpectedly, we identified that kinase heterodimers form structurally and functionally distinct complexes depending on the activation state of the MAPK. These may be incompatible with downstream signaling, but naturally they may also form structures that are compatible with the phosphorylation of the downstream kinase at the activation loop, or alternatively at other allosteric sites. Furthermore, we show that small-molecule inhibitors may affect the quaternary arrangement of kinase heterodimers and thus influence downstream signaling in a specific manner.

Theileria highjacks JNK2 into a complex with the macroschizont GPI (GlycosylPhosphatidylInositol)-anchored surface protein p104

Constitutive c-Jun N-terminal kinase (JNK) activity characterizes bovine T and B cells infected with Theileria parva, and B cells and macrophages infected with Theileria annulata. Here, we show that T. annulata infection of macrophages manipulates JNK activation by recruiting JNK2 and not JNK1 to the parasite surface, whereas JNK1 is found predominantly in the host cell nucleus. At the parasite's surface, JNK2 forms a complex with p104, a GPI-(GlycosylPhosphatidylInositol)-anchor T. annulata plasma membrane protein. Sequestration of JNK2 depended on Protein Kinase-A (PKA)-mediated phosphorylation of a JNK-binding motif common to T. parva and a cell penetrating peptide harbouring the conserved p104 JNK-binding motif competitively ablated binding, whereupon liberated JNK2 became ubiquitinated and degraded. Cytosolic sequestration of JNK2 suppressed small mitochondrial ARF-mediated autophagy, whereas it sustained nuclear JNK1 levels, c-Jun phosphorylation, and matrigel traversal. Therefore, T. annulata sequestration of JNK2 contributes to both survival and dissemination of Theileria-transformed macrophages.

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).

Dynamic control of RSK complexes by phosphoswitch-based regulation

Assembly and disassembly of protein-protein complexes needs to be dynamically controlled and phosphoswitches based on linear motifs are crucial in this process. Extracellular signal-regulated kinase 2 (ERK2) recognizes a linear-binding motif at the C-terminal tail (CTT) of ribosomal S6 kinase 1 (RSK1), leading to phosphorylation and subsequent activation of RSK1. The CTT also contains a classical PDZ domain-binding motif which binds RSK substrates (e.g. MAGI-1). We show that autophosphorylation of the disordered CTT promotes the formation of an intramolecular charge clamp, which efficiently masks critical residues and indirectly hinders ERK binding. Thus, RSK1 CTT operates as an autoregulated phosphoswitch: its phosphorylation at specific sites affects its protein-binding capacity and its conformational dynamics. These biochemical feedbacks, which form the structural basis for the rapid dissociation of ERK2-RSK1 and RSK1-PDZ substrate complexes under sustained epidermal growth factor (EGF) stimulation, were structurally characterized and validated in living cells. Overall, conformational changes induced by phosphorylation in disordered regions of protein kinases, coupled to allosteric events occurring in the kinase domain cores, may provide mechanisms that contribute to the emergence of complex signaling activities. In addition, we show that phosphoswitches based on linear motifs can be functionally classified as ON and OFF protein-protein interaction switches or dimmers, depending on the specific positioning of phosphorylation target sites in relation to functional linear-binding motifs. Moreover, interaction of phosphorylated residues with positively charged residues in disordered regions is likely to be a common mechanism of phosphoregulation.

DATABASE

Structural data are available in the PDB database under the accession numbers 5N7D, 5N7F and 5N7G. NMR spectral assignation data are available in the BMRB database under the accession numbers 27213 and 27214.

Systematic discovery of linear binding motifs targeting an ancient protein interaction surface on MAP kinases

Mitogen‐activated protein kinases (MAPK) are broadly used regulators of cellular signaling. However, how these enzymes can be involved in such a broad spectrum of physiological functions is not understood. Systematic discovery of MAPK networks both experimentally and in silico has been hindered because MAPKs bind to other proteins with low affinity and mostly in less‐characterized disordered regions. We used a structurally consistent model on kinase‐docking motif interactions to facilitate the discovery of short functional sites in the structurally flexible and functionally under‐explored part of the human proteome and applied experimental tools specifically tailored to detect low‐affinity protein–protein interactions for their validation in vitro and in cell‐based assays. The combined computational and experimental approach enabled the identification of many novel MAPK‐docking motifs that were elusive for other large‐scale protein–protein interaction screens. The analysis produced an extensive list of independently evolved linear binding motifs from a functionally diverse set of proteins. These all target, with characteristic binding specificity, an ancient protein interaction surface on evolutionarily related but physiologically clearly distinct three MAPKs (JNK, ERK, and p38). This inventory of human protein kinase binding sites was compared with that of other organisms to examine how kinase‐mediated partnerships evolved over time. The analysis suggests that most human MAPK‐binding motifs are surprisingly new evolutionarily inventions and newly found links highlight (previously hidden) roles of MAPKs. We propose that short MAPK‐binding stretches are created in disordered protein segments through a variety of ways and they represent a major resource for ancient signaling enzymes to acquire new regulatory roles.

Structural Basis of Ribosomal S6 Kinase 1 (RSK1) Inhibition by S100B Protein: MODULATION OF THE EXTRACELLULAR SIGNAL-REGULATED KINASE (ERK) SIGNALING CASCADE IN A CALCIUM-DEPENDENT WAY

Mitogen-activated protein kinases (MAPK) promote MAPK-activated protein kinase activation. In the MAPK pathway responsible for cell growth, ERK2 initiates the first phosphorylation event on RSK1, which is inhibited by Ca(2+)-binding S100 proteins in malignant melanomas. Here, we present a detailed in vitro biochemical and structural characterization of the S100B-RSK1 interaction. The Ca(2+)-dependent binding of S100B to the calcium/calmodulin-dependent protein kinase (CaMK)-type domain of RSK1 is reminiscent of the better known binding of calmodulin to CaMKII. Although S100B-RSK1 and the calmodulin-CAMKII system are clearly distinct functionally, they demonstrate how unrelated intracellular Ca(2+)-binding proteins could influence the activity of the CaMK domain-containing protein kinases. Our crystallographic, small angle x-ray scattering, and NMR analysis revealed that S100B forms a "fuzzy" complex with RSK1 peptide ligands. Based on fast-kinetics experiments, we conclude that the binding involves both conformation selection and induced fit steps. Knowledge of the structural basis of this interaction could facilitate therapeutic targeting of melanomas.

The NM23-H1/H2 homolog NDK-1 is required for full activation of Ras signaling in C. elegans

The group I members of the Nm23 (non-metastatic) gene family encode nucleoside diphosphate kinases (NDPKs) that have been implicated in the regulation of cell migration, proliferation and differentiation. Despite their developmental and medical significance, the molecular functions of these NDPKs remain ill defined. To minimize confounding effects of functional compensation between closely related Nm23 family members, we studied ndk-1, the sole Caenorhabditis elegans ortholog of group I NDPKs, and focused on its role in Ras/mitogen-activated protein kinase (MAPK)-mediated signaling events during development. ndk-1 inactivation leads to a protruding vulva phenotype and affects vulval cell fate specification through the Ras/MAPK cascade. ndk-1 mutant worms show severe reduction of activated, diphosphorylated MAPK in somatic tissues, indicative of compromised Ras/MAPK signaling. A genetic epistasis analysis using the vulval induction system revealed that NDK-1 acts downstream of LIN-45/Raf, but upstream of MPK-1/MAPK, at the level of the kinase suppressors of ras (KSR-1/2). KSR proteins act as scaffolds facilitating Ras signaling events by tethering signaling components, and we suggest that NDK-1 modulates KSR activity through direct physical interaction. Our study reveals that C. elegans NDK-1/Nm23 influences differentiation by enhancing the level of Ras/MAPK signaling. These results might help to better understand how dysregulated Nm23 in humans contributes to tumorigenesis.

Structural mechanism for the specific assembly and activation of the extracellular signal regulated kinase 5 (ERK5) module

Mitogen-activated protein kinase (MAPK) activation depends on a linear binding motif found in all MAPK kinases (MKK). In addition, the PB1 (Phox and Bem1) domain of MKK5 is required for extracellular signal regulated kinase 5 (ERK5) activation. We present the crystal structure of ERK5 in complex with an MKK5 construct comprised of the PB1 domain and the linear binding motif. We show that ERK5 has distinct protein-protein interaction surfaces compared with ERK2, which is the closest ERK5 paralog. The two MAPKs have characteristically different physiological functions and their distinct protein-protein interaction surface topography enables them to bind different sets of activators and substrates. Structural and biochemical characterization revealed that the MKK5 PB1 domain cooperates with the MAPK binding linear motif to achieve substrate specific binding, and it also enables co-recruitment of the upstream activating enzyme and the downstream substrate into one signaling competent complex. Studies on present day MAPKs and MKKs hint on the way protein kinase networks may evolve. In particular, they suggest how paralogous enzymes with similar catalytic properties could acquire novel signaling roles by merely changing the way they make physical links to other proteins.

Specificity of linear motifs that bind to a common mitogen-activated protein kinase docking groove

Mitogen-activated protein kinases (MAPKs) have a docking groove that interacts with linear "docking" motifs in binding partners. To determine the structural basis of binding specificity between MAPKs and docking motifs, we quantitatively analyzed the ability of 15 docking motifs from diverse MAPK partners to bind to c-Jun amino-terminal kinase 1 (JNK1), p38α, and extracellular signal-regulated kinase 2 (ERK2). Classical docking motifs mediated highly specific binding only to JNK1, and only those motifs with a sequence pattern distinct from the classical MAPK binding docking motif consensus differentiated between the topographically similar docking grooves of ERK and p38α. Crystal structures of four complexes of MAPKs with docking peptides, representing JNK-specific, ERK-specific, or ERK- and p38-selective binding modes, revealed that the regions located between consensus positions in the docking motifs showed conformational diversity. Although the consensus positions in the docking motifs served as anchor points that bound to common MAPK surface features and mostly contributed to docking in a nondiscriminatory fashion, the conformation of the intervening region between the anchor points mostly determined specificity. We designed peptides with tailored MAPK binding profiles by rationally changing the length and amino acid composition of intervening regions located between anchor points. These results suggest a coherent structural model for MAPK docking specificity that reveals how short linear motifs binding to a common kinase docking groove can mediate diverse interaction patterns and contribute to correct MAPK partner selection in signaling networks.

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.

Scaffolds are 'active' regulators of signaling modules

Signaling cascades, in addition to proteins with obvious signaling-relevant activities (e.g. protein kinases or receptors), also employ dedicated 'inactive' proteins whose functions appear to be the organization of the former components into higher order complexes through protein-protein interactions. The core function of signaling adaptors, anchors and scaffolds is the recruitment of proteins into one macromolecular complex. Several recent studies have demonstrated that the recruiter and the recruited molecules mutually influence each other in a scaffolded complex. This yields fundamentally novel properties for the signaling complex as a whole. Because these are not merely additive to the properties of the individual components, scaffolded signaling complexes may behave as functionally distinct modules.

Regulation of calpain B from Drosophila melanogaster by phosphorylation

Calpain B is one of the two catalytically competent calpain (calcium-activated papain) isoenzymes in Drosophila melanogaster. Because structural predictions hinted at the presence of several potential phosphorylation sites in this enzyme, we investigated the in vitro phosphorylation of the recombinant protein by protein kinase A as well as by the extracellular signal-regulated protein kinases (ERK) 1 and 2. By MS, we identified Ser845 in the Ca2+ binding region of an EF-hand motif, and Ser240 close to the autocatalytic activation site of calpain B, as being the residues phosphorylated by protein kinase A. In the transducer region of the protease, Thr747 was shown to be the target of the ERK phosphorylation. Based on the results of three different assays, we concluded that the treatment of calpain B with protein kinase A and ERK1 and ERK2 kinases increases the rate of the autoproteolytic activation of the enzyme, together with the rate of the digestion of external peptide or protein substrates. Phosphorylation also elevates the Ca2+ sensitivity of the protease. The kinetic analysis of phosphorylation mimicking Thr747Glu and Ser845Glu calpain B mutants confirmed the above conclusions. Out of the three phosphorylation events tested in vitro, we verified the in vivo phosphorylation of Thr747 in epidermal growth factor-stimulated Drosophila S2 cells. The data obtained suggest that the activation of the ERK pathway by extracellular signals results in the phosphorylation and activation of calpain B in fruit flies.

Multiple interactions of the 'transducer' govern its function in calpain activation by Ca2+

Typical calpains in mammals become activated on binding of 8-12 Ca2+ ions per enzyme molecule, giving an example of integrated, manifold regulation by calcium. Besides two identified Ca2+ sites in catalytic domain II and several EF-hand motifs in domains IV and VI, an acidic loop in the centrally positioned domain III seems to harbour Ca2+. The mediator of distant Ca2+-induced structural transitions is an elongated structural element, the 'transducer'. By site-directed mutagenesis along the transducer, we have generated various forms of rat m-calpain in which critical intramolecular interactions, as judged from the X-ray structure, would be abolished or modified. The kinetic parameters of these mutant enzymes support a model featuring shrinkage of transducer as a contributor to structural changes involved in calpain activation.

Contribution of Distinct Structural Elements to Activation of Calpain by Ca2+ Ions

The effect of Ca2+ in calpain activation is mediated via several binding sites in the enzyme molecule. To test the contribution of structural elements suspected to be part of this Ca2+ relay system, we made a site-directed mutagenesis study on calpains, measuring consequential changes in Ca2+ binding and Ca2+ sensitivity of enzyme activity. Evidence is provided for earlier suggestions that an acidic loop in domain III and the transducer region connecting domains III and IV are part of the Ca2+ relay system. Wild-type Drosophila Calpain B domain III binds two to three Ca2+ ions with a K(d) of 3400 microm. Phospholipids lower this value to 220 microm. Ca2+ binding decreases in parallel with the number of mutated loop residues. Deletion of the entire loop abolishes binding of the ion. The Ca2+ dependence of enzyme activity of various acidic-loop mutants of Calpain B and rat m-calpain suggests the importance of the loop in regulating activity. Most conspicuously, the replacement of two adjacent acidic residues in the N-terminal half of the loop evokes a dramatic decrease in the Ca2+ need of both enzymes, lowering half-maximal Ca2+ concentration from 8.6 to 1.3 mm for Calpain B and from 250 to 7 microm for m-calpain. Transducer-region mutations in m-calpain also facilitate Ca2+ activation with the most profound effect seen upon shortening the region by deletion mutagenesis. All of these data along with structural considerations suggest that the acidic loop and the transducer region form an interconnected, extended structural unit that has the capacity to integrate and transduce Ca2+-evoked conformational changes over a long distance. A schematic model of this "extended transducer" mechanism is presented.

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.