Phosphorylation of the Actin Binding Protein Drebrin at S647 Is Regulated by Neuronal Activity and PTEN

Defects in actin dynamics affect activity-dependent modulation of synaptic transmission and neuronal plasticity, and can cause cognitive impairment. A salient candidate actin-binding protein linking synaptic dysfunction to cognitive deficits is Drebrin (DBN). However, the specific mode of how DBN is regulated at the central synapse is largely unknown. In this study we identify and characterize the interaction of the PTEN tumor suppressor with DBN. Our results demonstrate that PTEN binds DBN and that this interaction results in the dephosphorylation of a site present in the DBN C-terminus - serine 647. PTEN and pS647-DBN segregate into distinct and complimentary compartments in neurons, supporting the idea that PTEN negatively regulates DBN phosphorylation at this site. We further demonstrate that neuronal activity increases phosphorylation of DBN at S647 in hippocampal neurons in vitro and in ex vivo hippocampus slices exhibiting seizure activity, potentially by inducing rapid dissociation of the PTEN:DBN complex. Our results identify a novel mechanism by which PTEN is required to maintain DBN phosphorylation at dynamic range and signifies an unusual regulation of an actin-binding protein linked to cognitive decline and degenerative conditions at the CNS synapse.     

Kindlin Binds Migfilin Tandem LIM Domains and Regulates Migfilin Focal Adhesion Localization and Recruitment Dynamics

Focal adhesions (FAs), sites of tight adhesion to the extracellular matrix, are composed of clusters of transmembrane integrin adhesion receptors and intracellular proteins that link integrins to the actin cytoskeleton and signaling pathways. Two integrin-binding proteins present in FAs, kindlin-1 and kindlin-2, are important for integrin activation, FA formation, and signaling. Migfilin, originally identified in a yeast two-hybrid screen for kindlin-2-interacting proteins, is a LIM domain-containing adaptor protein found in FAs and implicated in control of cell adhesion, spreading, and migration. By binding filamin, migfilin provides a link between kindlin and the actin cytoskeleton. Here, using a combination of kindlin knockdown, biochemical pulldown assays, fluorescence microscopy, fluorescence resonance energy transfer (FRET), and fluorescence recovery after photobleaching (FRAP), we have established that the C-terminal LIM domains of migfilin dictate its FA localization, shown that these domains mediate an interaction with kindlin in vitro and in cells, and demonstrated that kindlin is important for normal migfilin dynamics in cells. We also show that when the C-terminal LIM domain region is deleted, then the N-terminal filamin-binding region of the protein, which is capable of targeting migfilin to actin-rich stress fibers, is the predominant driver of migfilin localization. Our work details a correlation between migfilin domains that drive kindlin binding and those that drive FA localization as well as a kindlin dependence on migfilin FA recruitment and mobility. We therefore suggest that the kindlin interaction with migfilin LIM domains drives migfilin FA recruitment, localization, and mobility.     

Deposition of lignin droplets produced during dilute acid pretreatment of maize stems retards enzymatic hydrolysis of cellulose

Electron microscopy of lignocellulosic biomass following high-temperature pretreatment revealed the presence of spherical formations on the surface of the residual biomass. The hypothesis that these droplet formations are composed of lignins and possible lignin carbohydrate complexes is being explored. Experiments were conducted to better understand the formation of these "lignin" droplets and the possible implications they might have on the enzymatic saccharification of pretreated biomass. It was demonstrated that these droplets are produced from corn stover during pretreatment under neutral and acidic pH at and above 130 degrees C, and that they can deposit back onto the surface of residual biomass. The deposition of droplets produced under certain pretreatment conditions (acidic pH; T > 150 degrees C) and captured onto pure cellulose was shown to have a negative effect (5-20%) on the enzymatic saccharification of this substrate. It was noted that droplet density (per unit area) was greater and droplet size more variable under conditions where the greatest impact on enzymatic cellulose conversion was observed. These results indicate that this phenomenon has the potential to adversely affect the efficiency of enzymatic conversion in a lignocellulosic biorefinery.     

Spontaneous pulmonary adenocarcinoma and subcutaneous cavernous hemangiomas arising in a squirrel monkey (Saimiri sciureus)

Benign and malignant pulmonary tumors have been reported in both Old World and New World monkeys but are uncommon. Hemangiomas are also rarely reported in nonhuman primates. Here we present a case of two primary neoplasms (a papillary adenocarcinoma of bronchioloalveolar origin and multiple cavernous subcutaneous hemangiomas) arising in an aged squirrel monkey (Saimiri sciureus).     

Protein kinase-independent activation of CFTR by phosphatidylinositol phosphates

The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel that is expressed in many epithelia and in the heart. Phosphorylation of CFTR by protein kinases is thought to be an absolute prerequisite for the opening of CFTR channels. In addition, nucleoside triphosphates were shown to regulate the opening of phosphorylated CFTR. Here, we report that phosphatidylinositol 4,5-bisphosphate (PIP₂) activates human CFTR, resulting in ATP responsiveness of PIP₂-treated CFTR. PIP₂ alone is not sufficient to open CFTR, but ATP opens nonphosphorylated CFTR after application of PIP₂. The effect of PIP₂ is independent of protein kinases, as PIP₂ activates CFTR in the complete absence of Mg. Phosphatidylinositol and phosphatidylinositol monophosphate activate CFTR less efficiently than PIP₂. PIP₂ application to phosphorylated CFTR may inhibit the CFTR chloride current. We suggest that regulation of CFTR by PIP₂ is a previously unrecognized, alternative mechanism to control chloride conductance.     

Crystal structure of scallop Myosin s1 in the pre-power stroke state to 2.6 a resolution: flexibility and function in the head

We have extended the X-ray structure determination of the complete scallop myosin head in the pre-power stroke state to 2.6 A resolution, allowing an atomic comparison of the three major (weak actin binding) states of various myosins. We can now account for conformational differences observed in crystal structures in the so-called "pliant region" at the motor domain-lever arm junction between scallop and vertebrate smooth muscle myosins. A hinge, which may contribute to the compliance of the myosin crossbridge, has also been identified for the first time within the regulatory light-chain domain of the lever arm. Analysis of temperature factors of key joints of the motor domain, especially the SH1 helix, provides crystallographic evidence for the existence of the "internally uncoupled" state in diverse isoforms. The agreement between structural and solution studies reinforces the view that the unwinding of the SH1 helix is a part of the cross-bridge cycle in many myosins.     

Site-directed perturbation of protein kinase C- integrin interaction blocks carcinoma cell chemotaxis

Polarized cell movement is an essential requisite for cancer metastasis; thus, interference with the tumor cell motility machinery would significantly modify its metastatic behavior. Protein kinase C alpha (PKC alpha) has been implicated in the promotion of a migratory cell phenotype. We report that the phorbol ester-induced cell polarization and directional motility in breast carcinoma cells is determined by a 12-amino-acid motif (amino acids 313 to 325) within the PKC alpha V3 hinge domain. This motif is also required for a direct association between PKC alpha and beta 1 integrin. Efficient binding of beta 1 integrin to PKC alpha requires the presence of both NPXY motifs (Cyto-2 and Cyto-3) in the integrin distal cytoplasmic domains. A cell-permeant inhibitor based on the PKC-binding sequence of beta 1 integrin was shown to block both PKC alpha-driven and epidermal growth factor (EGF)-induced chemotaxis. When introduced as a minigene by retroviral transduction into human breast carcinoma cells, this inhibitor caused a striking reduction in chemotaxis towards an EGF gradient. Taken together, these findings identify a direct link between PKC alpha and beta 1 integrin that is critical for directed tumor cell migration. Importantly, our findings outline a new concept as to how carcinoma cell chemotaxis is enhanced and provide a conceptual basis for interfering with tumor cell dissemination.     

Energetics for displacing a single chain from the surface of microcrystalline cellulose into the active site of Acidothermus cellulolyticus Cel5A

A series of molecular mechanics calculations were used to analyze the energetics for moving a single polysaccharide chain from the surface of microcrystalline cellulose into the binding cleft of the Cel5A cellulase from Acidothermus cellulolyticus. A build-up procedure was used to model the placement of a 12-residue oligosaccharide chain along the surface of the enzyme, using as a guide the four residues of the tetrasaccharide substrate co-crystallized with the protein in the crystallographic structure determination. The position of this 12-residue oligosaccharide was used to orient the enzyme properly above two different surfaces of cellulose 1beta, the (1,0,0) and the (1,1,0) faces of the crystal. Constrained molecular dynamics simulations were then used to pull a target chain directly below the enzyme up out of the crystal surface and into the binding groove. The energetics for this process were favorable for both faces, with the step face being more favorable than the planar face, implying that this surface could be hydrolyzed more readily.     

Computational and experimental studies of the catalytic mechanism of Thermobifida fusca cellulase Cel6A (E2)

Mutagenesis experiments suggest that Asp79 in cellulase Cel6A (E2) from Thermobifida fusca has a catalytic role, in spite of the fact that this residue is more than 13 A from the scissile bond in models of the enzyme-substrate complex built upon the crystal structure of the protein. This suggests that there is a substantial conformational shift in the protein upon substrate binding. Molecular mechanics simulations were used to investigate possible alternate conformations of the protein bound to a tetrasaccharide substrate, primarily involving shifts of the loop containing Asp79, and to model the role of water in the active site complex for both the native conformation and alternative low-energy conformations. Several alternative conformations of reasonable energy have been identified, including one in which the overall energy of the enzyme-substrate complex in solution is lower than that of the conformation in the crystal structure. This conformation was found to be stable in molecular dynamics simulations with a cellotetraose substrate and water. In simulations of the substrate complexed with the native protein conformation, the sugar ring in the -1 binding site was observed to make a spontaneous transition from the (4)C(1) conformation to a twist-boat conformer, consistent with generally accepted glycosidase mechanisms. Also, from these simulations Tyr73 and Arg78 were found to have important roles in the active site. Based on the results of these various MD simulations, a new catalytic mechanism is proposed. Using this mechanism, predictions about the effects of changes in Arg78 were made which were confirmed by site-directed mutagenesis.     

Survey and analysis of commercial cellulase preparations suitable for biomass conversion to ethanol

Commercial cellulase enzymes have been used in the food, detergent, and textile industries, and are potentially effective for processing biomass feedstocks. A survey was undertaken to identify major manufacturers/distributors of cellulases in the USA and to evaluate 13 representative commercial preparations for enzyme activity, protein concentration, and chemical composition. Samples were subjected to activity measurements using filter paper, carboxymethylcellulose, cellobiose, and p-nitrophenyl--d-glucopyranoside as substrates. To ascertain the microbial origin of the commercial preparations, Western blots utilizing monoclonal antibodies specific for Trichoderma reesei CBH I and Aspergillus niger -d-glucosidase were developed. Eleven of the cellulases tested were of T. reesei or T. viride origin and two were from A. niger.