Actin-stimulated myosin Mg2+-ATPase inhibition by brain protein

The actin depolymerizing factor (ADF)/cofilin signaling pathway and DNA damage responses in cancer

The actin depolymerizing factor (ADF)/cofilin protein family is essential for actin dynamics, cell division, chemotaxis and tumor metastasis. Cofilin-1 (CFL-1) is a primary non-muscle isoform of the ADF/cofilin protein family accelerating the actin filamental turnover in vitro and in vivo. In response to environmental stimulation, CFL-1 enters the nucleus to regulate the actin dynamics. Although the purpose of this cytoplasm-nucleus transition remains unclear, it is speculated that the interaction between CFL-1 and DNA may influence various biological responses, including DNA damage repair. In this review, we will discuss the possible involvement of CFL-1 in DNA damage responses (DDR) induced by ionizing radiation (IR), and the implications for cancer radiotherapy.

Cloning and characterization of actin depolymerizing factor from Toxoplasma gondii

We determined the predicted amino acid sequence of actin depolymerizing factor (ADF) from Toxoplasma gondii by sequencing the full-length cDNA. T. gondii ADF consists of 118 amino acids (calculated molecular weight 13,400) and shares a high degree of sequence similarity to other low molecular weight actin monomer sequestering proteins, especially Acanthamoeba actophorin, plant ADFs and yeast and vertebrate cofilin. ADF from T. gondii is smaller and does not contain a nuclear localization sequence like the related vertebrate proteins. Southern blot analysis indicates that T. gondii ADF is a single-copy gene. Homogeneous recombinant T. gondii ADF purified from E. coli is active in binding actin monomers and depolymerizing F-actin. Localization of ADF by immunofluorescence and immunoelectron microscopy indicates ADF is scattered throughout the cytoplasm and prominently localized beneath the plasma membrane in T. gondii.

Analysis of the interactions of actin depolymerizing factor with G- and F-actin

Chick actin depolymerizing factor (ADF) is an actin binding protein previously shown to rapidly depolymerize actin filaments in vitro, yielding a 1:1 complex of ADF and actin monomer. Here we show that ADF protects actin monomer from denaturation by EDTA by inhibiting the exchange of actin-bound nucleotide. Under low ionic strength conditions, the approximate dissociation constant (KD) for the ADF-actin complex determined from exchange of nucleotide (1,N6-etheno-ATP) is about 150 and is calcium-independent. Addition of ADF to monomeric actin inhibits actin assembly as well as the ATP hydrolysis that normally accompanies assembly. Complex formation is demonstrated between ADF and actin containing either ATP, ADP, or AMPPNP as the bound nucleotide. A KD of 0.1-0.2 microM was calculated for both the ADF-ATP-actin and ADF-AMPPNP-actin complexes, whereas the KD for the ADF-ADP-actin complex is about 1.3 microM. ADF can either depolymerize or cosediment with F-actin in a stoichiometric fashion, but these reciprocal activities are pH-dependent. At pHs between 6.5 and 7.1, ADF cosediments with F-actin and demonstrates only weak depolymerizing activity. ADF binding is cooperative and saturates at a 1:1 ADF:actin molar ratio. At pHs between 7.1 and 7.7, ADF shows increasing depolymerizing activity and less F-actin binding. At pH 8.0, ADF depolymerizes F-actin in a stoichiometric manner. Both the F-actin binding and the depolymerizing activities of ADF are inhibited by phalloidin.(ABSTRACT TRUNCATED AT 250 WORDS)

The interaction of 6-propionyl-2-(NN-dimethyl)aminonaphthalene (PRODAN)-labelled actin with actin-binding proteins and drugs

The influence of various actin-binding proteins and drugs on the fluorescence emission of rabbit muscle actin labelled with the fluorescent probe acrylodan (6-acryloyl-2-dimethylaminonaphthalene) at Cys-374, the penultimate amino acid residue of the actin amino acid sequence, was studied. Addition of myosin, tropomyosin or phalloidin, agents known to bind only to filamentous F-actin, did not change the emission energy or the integrated intensity of the fluorescence spectrum. The presence of heavy meromyosin or of the glycolytic enzyme aldolase led to a small (approx. 2%) increase in the integrated intensity, and in the energy of the emitted fluorescence. The interaction of 6-propionyl-2-(NN-dimethyl)aminonaphthalene (PRODAN)-F-actin with pancreatic DNAase I and with a filament-severing 19 kDa protein from pig brain resulted in the gradual reduction of the integrated intensity of the emission and a red shift of the emission energy, suggestive of a disintegration of the actin filament structure. Profilin caused a < 10% change in the emission energy. Cytochalasin D reduced the integrated intensity of PRODAN-F-actin and red-shifted the emission energy, while cytochalasin B was without influence. Pancreatic DNAase I did not change the fluorescence emission of PRODAN-G-actin, suggesting that binding of this enzyme does not alter the environment of the probe. When the 19 kDa protein bound to PRODAN-G-actin, however, the integrated intensity was reduced and the emission energy was lowered. This effect was exploited to estimate the binding constant for the interaction between the 19 kDa protein and PRODAN-G-actin. The Kd was found to be about 0.25 microM.

Identification of two species of actin depolymerizing factor in cultures of BHK cells

High-speed supernatant obtained from the lysate of cultured BHK cells has been chromatographed on Sepharose-4B, DEAE-cellulose and hydroxyapatite columns, and a fraction has been identified with characteristics similar to an actin depolymerizing factor (ADF), a small protein previously isolated from embryonic chick brain. Using a rabbit antibody against the chick brain protein, two immunoreactive forms were identified: a 19 kDa form co-migrating in SDS-polyacrylamide gels with embryonic chick brain ADF, and a 20 kDa form. The two species could be separated on a hydroxyapatite or green A dye matrix columns and only the 20 kDa protein was active when assayed for effects on pyrene-G-actin assembly. It enhanced the rate of F-actin assembly, but only after an initial lag phase, and decreased the final proportion of actin in filamentous form. These effects were calcium-independent. Actin depolymerizing factor constituted at least 0.5% of the total protein in the cytoplasmic fraction. A Triton extract of plasma membrane-enriched fraction from BHK cells was fractionated on a Sepharose-4B column and again, a fraction was found which had an ADF-like activity and also contained the two immuno-cross-reactive forms, 19 kDa and 20 kDa. These results suggest a novel regulation of the microfilament system in eukaryotic cells via the control of the ADF activity.

Distribution and cellular localization of actin depolymerizing factor

Actin depolymerizing factor (ADF) is a low molecular mass (19 kD) protein that forms a tightly bound dimeric complex with actin. We have raised a rabbit antiserum to chick brain ADF and used it to analyze the distribution and cellular localization of ADF. We find that ADF is a major constituent of all chick embryonic and most adult tissues examined, accounting for 0.1-0.4% of the total protein. Some tissues have as much as 0.6 mol ADF per mole actin. Adult heart and skeletal muscle are unusual in having very low levels of ADF: less than 0.02% of the soluble protein. During the development of skeletal muscle, ADF levels are maximal up to approximately 11 d in ovo and then decline to reach their adult levels by 14 d posthatching. Brain tissue and cultured cell lines from several other vertebrates, including mammals, all possess proteins of identical size to ADF that are recognized by the ADF antiserum. No proteins are specifically recognized by the ADF antiserum in extracts from Acanthamoeba castellanii or from nerve tissue of several invertebrates. Indirect immunofluorescence shows that ADF is present throughout the cytosol of most cells and at the leading edge of ruffled membranes and in the neuronal growth cone. Its abundance and widespread distribution together with its ability to sequester actin molecules, even those in an already polymerized state, suggest that ADF is a major factor in the regulation of actin filaments in many vertebrate cells.

A possible mechanism of morphometric changes in dendritic spines induced by stimulation

A number of experimental procedures which induce increased electrical activity (including long-term potentiation) were shown to be accompanied by morphometric changes in dendritic spines. These changes include an enlargement of the spine head, shortening and widening of the spine stalk, and an increase in the length of synaptic apposition. A possible mechanism is suggested which takes into account specific cytological features of the spine and the existence of contractile proteins in neurons. Dendritic spines are defined as special domains of the neuron which have a unique organization of the cytoplasm. Actin filaments form a very dense network in the spine head, and they are longitudinally organized within the spine stalk. Spines were also shown to contain myosin and other actin-regulatory proteins. The high density of the actin network could explain the characteristic absence of the cytoplasmic organelles from dendritic spines. In analogy with other cells, such an actin organization indicates low levels of free cytosolic calcium. Even in the resting state, calcium levels may be unevenly distributed through the neuron, being lowest within the subplasmalemmal region. Due to the high surface-to-volume ratio in spines, the cytoplasm is formed mostly by the subplasmalemmal region. The spine apparatus or the smooth endoplasmic reticulum, which is recognized as a calcium-sequestering site in spines, may also contribute to the low calcium levels there. However, when in the stimulated spine the voltage-dependent calcium channels open, then, given the spine's high surface-to-volume ratio, the concentration of calcium may very quickly attain levels that will activate the actin-regulatory proteins and myosin and thus trigger the chain of events leading to the enlargement of the spine head and to the contraction (i.e., widening and shortening) of the spine stalk. The increased free cytosolic calcium may also activate the protein-producing system localized at the base of the spine, which, under certain conditions, could stabilize the morphometric changes of the spine.