Search PubMed⌕ Search

Biomedical subjects

V Bennett

Publications and source records attributed to V Bennett.

166 records · Page 10Linked to original sources
PubMedDate not supplied

Synapsin I is a microtubule-bundling protein.

Synapsin I, a synaptic vesicle protein, is thought to be involved in the regulation of neurotransmission through its phosphorylation by the cyclic AMP-dependent and Ca2+/calmodulin-dependent protein kinases which become activated upon depolarization of nerve endings. However, despite its recent characterization as a spectrin-binding protein immunologically related to erythrocyte protein 4.1, other interactions of synapsin I with structural proteins remain unknown. We report here that synapsin I can co-cycle with microtubules through three cycles of warm polymerization and cold depolymerization. Synapsin I binds saturably to microtubules stabilized by taxol, with an estimated dissociation constant (Kd) of 4.5 microM and a stoichiometry of 1.2 mol of synapsin binding sites per mol tubulin dimer. Synapsin I also increases the turbidity of tubulin solutions at 37 degrees C, but without causing detectable alterations in the critical concentration required for polymerization. Mixtures of synapsin I and tubulin observed by negative stain electron microscopy contain bundles of microtubules, accounting for the effect of synapsin I on tubulin turbidity. Synapsin I is thus a candidate to mediate or regulate the interaction of synaptic vesicles with microtubules.

Alkaloids↗
PubMedDate not supplied

Modulation of spectrin-actin assembly by erythrocyte adducin.

The spectrin-based membrane skeleton, an assembly of proteins tightly associated with the plasma membrane, determines the shape and mechanical properties of erythrocytes. Spectrin, the most abundant component of this assembly, is an elongated and flexible molecule that, with potentiation by protein 4.1, is cross-linked at its ends by short actin filaments to form a lattice beneath the membrane. These and other proteins stabilize the plasma membrane, organize integral membrane proteins and maintain specialized regions of the cell surface. A membrane-skeleton-associated calmodulin-binding protein of erythrocytes is a major substrate for Ca2+- and phospholipid-dependent protein kinase C (ref. 5), and thus is a target for Ca2+ by two regulatory pathways. Here we demonstrate that this protein, called adducin: (1) binds tightly in vitro to spectrin-actin complexes but with much less affinity either to spectrin or to actin alone; (2) promotes assembly of additional spectrin molecules onto actin filaments; and (3) is inhibited in its ability to induce the binding of additional spectrin molecules to actin by micromolar concentrations of calmodulin and Ca2+. Adducin may be involved in the action of Ca2+ on erythrocyte membrane skeleton and in the assembly of spectrin-actin complexes.

Actins↗
PubMedDate not supplied

Kinetics of irreversible activation of adenylate cyclase of fat cell membranes by phosphonium and phosphoramidate analogs of gtp1.

The ability of guanylylimidodiphosphate (GMP=P(NH)P) and guanylylmethylenediphosphonate (GMP-P(CH2)P) to activate adenylate cyclase activity has been studied by incubating these analogs with fat cell membranes followed by thorough washing of the membranes before assay of enzyme activity. GMP-P(NH)P is hydrolyzed by membrane preparations from several tissues. A pyruvate kinase regenerating system maintains the concentration of GMP-P(NH)P and thereby augments the ability of suboptimal concentrations of GMP-P(NH)P to activate adenylate cyclase. GTP inhibits activation of fat cell membrane adenylate cyclase by GMP-P(NH)P but this inhibition is overcome by time. This is consistent with the virtually irreversible nature of the GMP-P(NH)P activation, and with the inability of GTP to reverse the stimulated state of the enzyme. Although the initial rate of enzyme activation is highly dependent on the concentration of GMP-P(NH)P, with increasing times of incubation nearly the same maximal extent of activation is seen over a wide range of concentrations. Thus, it is not possible to estimate true affinity constants (at equilibrium) for GMP-P(NH)P, as anticipated from the virtually irreversible character of the activation process.

Adenylyl Cyclases↗