Covalent modification of G-actin by pyridoxal 5'-phosphate: polymerization properties and interaction with DNase I and myosin subfragment 1

Pyridoxal-5'-phosphate phosphatase/chronophin inhibits long-term potentiation induction in the rat dentate gyrus

Pyridoxal-5'-phosphate (PLP)-phosphatase/chronophin (PLPP/CIN) directly dephosphorylates actin-depolymerizing factor (ADF)/cofilin as well as PLP. Although PLPP/CIN plays a role in the regulation of F-actin and vitamin B(6) metabolism, there is no direct evidence to support a correlation between PLPP/CIN and F-actin polymerization during long-term potentiation (LTP) induction. In this study, we investigated whether the expression of PLPP/CIN is altered following LTP induction, and whether Tat-PLPP/CIN transduction affects LTP induction in the rat dentate gyrus (DG). PLPP/CIN immunoreactivity was markedly decreased in dentate granule cells after the induction of LTP. Tat-PLPP/CIN transduction (20 and 200 microg/kg) decreased the efficiency of high frequency stimulus-induced potentiation of populations spike amplitude as compared to saline or Tat-protein-treated animals. The PLPP/CIN protein level showed an inverse correlation with phosphorylated ADF/cofilin levels and F-actin content. These findings suggest that PLPP/CIN-mediated actin dynamics may play an important role in the changes of morphological properties (dendritic spine reorganization) of the hippocampus in LTP.

Enhanced pyridoxal 5'-phosphate synthetic enzyme immunoreactivities do not contribute to GABAergic inhibition in the rat hippocampus following pilocarpine-induced status epilepticus

To comprehend the role of pyridoxal 5'-phosphate (PLP) in epilepsy or seizure, we investigated whether the expressions of two PLP synthetic enzymes (pyridoxal kinase, PLK; pyridoxine-5'-phosphate oxidase, PNPO) are altered in the hippocampus and whether changes in paired-pulse responses in the hippocampus are associated with altered PLP synthetic enzyme expressions following status epilepticus (SE). PLK and PNPO immunoreactivities were significantly increased in the rat hippocampus accompanied by reductions in paired-pulse inhibition at 1 day and 1 week after SE. Four weeks after SE, PLK and PNPO immunoreactivities in dentate granule cells were similar to those in control animals, while their immunoreactivities were markedly reduced in Cornu Ammonis 1 (CA1) pyramidal cells due to neuronal loss. Linear regression analysis identified a direct proportional relationship between PLK/PNPO immunoreactivity and normalized population spike amplitude ratio in the dentate gyrus and the CA1 region as excluded the data obtained from 4 weeks after SE. These findings indicate that the upregulation of PLK and PNPO immunoreactivities in principal neurons may not be involved in gamma-aminobutyric acid (GABA)ergic inhibition, but rather in enhanced excitability during epileptogenic periods.

Phosphorylation of actin Tyr-53 inhibits filament nucleation and elongation and destabilizes filaments

Dictyostelium actin was shown to become phosphorylated on Tyr-53 late in the developmental cycle and when cells in the amoeboid stage are subjected to stress but the phosphorylated actin had not been purified and characterized. We have separated phosphorylated and unphosphorylated actin and shown that Tyr-53 phosphorylation substantially reduces actin's ability to inactivate DNase I, increases actin's critical concentration, and greatly reduces its rate of polymerization. Tyr-53 phosphorylation substantially, if not completely, inhibits nucleation and elongation from the pointed end of actin filaments and reduces the rate of elongation from the barbed end. Negatively stained electron microscopic images of polymerized Tyr-53-phosphorylated actin show a variable mixture of small oligomers and filaments, which are converted to more typical, long filaments upon addition of myosin subfragment 1. Tyr-53-phosphorylated and unphosphorylated actin copolymerize in vitro, and phosphorylated and unphosphorylated actin colocalize in amoebae. Tyr-53 phosphorylation does not affect the ability of filamentous actin to activate myosin ATPase.

Thymosin beta 4 interactions

Thymosin beta 4 is a small, 5-kDa protein with a diverse range of activities, including its function as an actin monomer sequestering protein, an antiinflammatory agent, and an inhibitor of bone marrow stem cell proliferation. Only the effects of thymosin beta 4 on the actin cytoskeleton have an explanation based on identified molecular interactions. Thymosin beta 4 is largely unfolded or perhaps completely unfolded in solution. Based on the paradigm introduced by Wright and Dyson (1999) that unfolded proteins may have multiple functions based on their ability to recognize numerous ligands, the flexible structure of thymosin beta 4 may facilitate the recognition of a variety of molecular targets, thus explaining the plethora of functions attributed to thymosin beta 4. Furthermore, if multiple ligands bind to thymosin beta 4, then it is possible that thymosin beta 4 has a unique integrative function that links the actin cytoskeleton to important immune and cell growth-signaling cascades.

Kinetics of the interaction of myosin subfragment-1 with G-actin. Effect of nucleotides and DNaseI

The kinetics of interaction of monomeric pyrenyl-labeled G-actin with myosin subfragment-1 (S1 (A1) and S1(A2) isomers) has been examined in the stopped-flow at low ionic strength. The data confirm the previously reported existence of binary GS and ternary G2S complexes. The increase in pyrenyl-actin fluorescence which monitors the G-actin-S1 interactions is linked to the isomerization of these complexes following rapid equilibrium binding steps. The rates of isomerization are approximately 200 s-1 for GS and approximately 50 s-1 for G2S at 4 degrees C and in the absence of ATP. DNaseI and S1 bind G-actin essentially in a mutually exclusive fashion. Both GS and G2S are dissociated by MgATP and MgADP. The kinetics and mechanism of ATP-induced dissociation of G2S are quantitatively close to the ATP-induced dissociation of F-actin-S1, which indicates the G2S is a good model for the F-actin-S1 interface. GS and G2S display different kinetic behaviors in response to nucleotides, GS being less efficiently dissociated than G2S by MgATP. This result suggests that different mechanical properties of the cross-bridge might correlate with different orientations of the myosin head and different actin/myosin binding ratios.

Actin and the actomyosin interface: a review

This review deals with the structure of the actin monomer, its assembly into filaments and the loci on F-actin involved in binding myosin. Two distinctly different arrangements of monomers have been suggested for actin filaments. One model proposed by Holmes et al. is well developed. It places the so-called 'large' domain close to the filament axis and the so-called 'small' domain out near the surface of the filament. A second, less-well developed, model proposed by Schutt et al. locates the 'small' domain close to the filament axis and they rotate the monomer so that 'bottom' of the 'large' domain is at the highest radius. We analyze the available evidence for the models of F-actin derived from X-ray diffraction, reconstructions from electron micrographs, fluorescence resonance energy transfer spectroscopy, chemical cross-linking, antibody probes, limited proteolysis, site-directed and natural mutations, nuclear magnetic resonance spectroscopy and other techniques. The result is an actin-centered view of the loci on actin which are probably involved in its interaction with the myosin 'head'. From these multiple contacts we speculate on the sequence of steps between the initial weak-binding state of S-1 to the actin filament through to the stable strong-binding state seen in the absence of free Mg-ATP, i.e., the rigor state.