It has been shown recently that electrically induced DNA transfer into cells is a fast vectorial process with the same direction as DNA electrophoresis in an external electric field (Klenchin, V. A., S. I. Sukharev, S. M. Serov, L. V. Chernomordik, and Y. A. Chizmadzhev. 1991. Biophys. J. 60:804-811). Here we describe the effect of DNA interaction with membrane electropores and provide additional evidences for the key role of DNA electrophoresis in cell electrotransfection. The assay of electrically induced uptake of fluorescent dextrans (FDs) by cells shows that the presence of DNA in the medium during electroporation leads to a sharp increase in membrane permeability to FDs of M(r) < 20,000. The permeability increases with DNA concentration and the effect is seen even if FD is added to the cell suspension a few minutes after pulse application. The longer the DNA fragment, the greater the increase in permeability. The use of a two-pulse technique allows us to separate two effects provided by a pulsed electric field: membrane electroporation and DNA electrophoresis. The first pulse (6 kV/cm, 10 microseconds) creates pores efficiently, whereas transfection efficiency (TE) is low. The second pulse of much lower amplitude, but substantially longer (0.2 kV/cm, 10 ms), does not cause poration and transfection by itself but enhances TE by about one order of magnitude. In two-pulse experiments, TE rises monotonously with the increase of the second pulse duration. By varying the delay duration between the two pulses, we estimate the lifetime of electropores (which are DNA-permeable in conditions of low electric field) as tens of seconds. The data suggest that the mechanism of cell electrotransfection is underlain by electrophoretic movement of DNA through membrane pores, the size of which is determined by interaction with DNA in an electric field.

Disruption of the presynaptically enriched polyphosphoinositide phosphatase synaptojanin 1 leads to an increase of clathrin-coated intermediates and of polymerized actin at endocytic zones of nerve terminals. These changes correlate with elevated levels of PI(4,5)P(2) in neurons. We report that phosphatidylinositol phosphate kinase type Igamma (PIPKIgamma), a major brain PI(4)P 5-kinase, is concentrated at synapses. Synaptojanin 1 and PIPKIgamma antagonize each other in the recruitment of clathrin coats to lipid membranes. Like synaptojanin 1 and other proteins involved in endocytosis, PIPKIgamma undergoes stimulation-dependent dephosphorylation. These results implicate PIPKIgamma in the synthesis of a PI(4,5)P(2) pool that acts as a positive regulator of clathrin coat recruitment and actin function at the synapse.

Marine macrolide toxins of trisoxazole family target actin with high affinity and specificity and have promising pharmacological properties. We present X-ray structures of actin in complex with two members of this family, kabiramide C and jaspisamide A, at a resolution of 1.45 and 1.6 A, respectively. The structures reveal the absolute stereochemistry of these toxins and demonstrate that their trisoxazole ring interacts with actin subdomain 1 while the aliphatic side chain is inserted into the hydrophobic cavity between actin subdomains 1 and 3. The binding site is essentially the same as the one occupied by the actin-capping domain of the gelsolin superfamily of proteins. The structural evidence suggests that actin filament severing and capping by these toxins is also analogous to that of gelsolin. Consequently, these macrolides may be viewed as small molecule biomimetics of an entire class of actin-binding proteins.

CAPS-1 is required for Ca2+-triggered fusion of dense-core vesicles with the plasma membrane, but its site of action and mechanism are unknown. We analyzed the kinetics of Ca2+-triggered exocytosis reconstituted in permeable PC12 cells. CAPS-1 increased the initial rate of Ca2+-triggered vesicle exocytosis by acting at a rate-limiting, Ca2+-dependent prefusion step. CAPS-1 activity depended upon prior ATP-dependent priming during which PIP2 synthesis occurs. CAPS-1 activity and binding to the plasma membrane depended upon PIP2. Ca2+ was ineffective in triggering vesicle fusion in the absence of CAPS-1 but instead promoted desensitization to CAPS-1 resulting from decreased plasma membrane PIP2. We conclude that CAPS-1 functions following ATP-dependent priming as a PIP2 binding protein to enhance Ca2+-dependent DCV exocytosis. Essential prefusion steps in dense-core vesicle exocytosis involve sequential ATP-dependent synthesis of PIP2 and the subsequent PIP2-dependent action of CAPS-1. Regulation of PIP2 levels and CAPS-1 activity would control the secretion of neuropeptides and monoaminergic transmitters.

Natural small-molecule inhibitors of actin cytoskeleton dynamics have long been recognized as valuable molecular probes for dissecting complex mechanisms of cellular function. More recently, their potential use as chemotherapeutic drugs has become a focus of scientific investigation. The primary focus of this review is the molecular mechanism by which different actin-targeting natural products function, with an emphasis on structural considerations of toxins for which high-resolution structural information of their interaction with actin is available. By comparing the molecular interactions made by different toxin families with actin, the structural themes of those that alter filament dynamics in similar ways can be understood. This provides a framework for novel synthetic-compound designs with tailored functional properties that could be applied in both research and clinical settings.

Yeast phosphatidylinositol transfer protein (Sec14p) is essential for Golgi secretory function. It is widely accepted, though unproven, that phosphatidylinositol transfer between membranes represents the physiological activity of phosphatidylinositol transfer proteins (PITPs). We report that Sec14pK66,239A is inactivated for phosphatidylinositol, but not phosphatidylcholine (PC), transfer activity. As expected, Sec14pK66,239A fails to meet established criteria for a PITP in vitro and fails to stimulate phosphoinositide production in vivo. However, its expression efficiently rescues the lethality and Golgi secretory defects associated with sec14-1ts and sec14 null mutations. This complementation requires neither phospholipase D activation nor the involvement of a novel class of minor yeast PITPs. These findings indicate that PI binding/transfer is remarkably dispensable for Sec14p function in vivo.

Simian Cos-1 cells were transfected electrically with the plasmid pCH110 carrying the beta-galactosidase gene. The efficiency of transfection was determined by a transient expression of this gene. When the plasmid was introduced into a cell suspension 2 s after pulse application, the transfection efficiency was shown to be less than 1% as compared with a prepulse addition of DNA. Addition of DNAase to suspension immediately after a pulse did not decrease transfection efficiency, thus the time of DNA translocation was estimated to be less than 3 s. The use of electric treatment medium, in which the postpulse colloid-osmotic cell swelling was prevented, did not affect the transfection efficiency. These results contradict both assumptions of free DNA diffusion into cell through the long-lived pores and of involvement of osmotic effects in DNA translocation. Transfection of cells in monolayer on a porous film allowed creation of the spatial asymmetry of cell-plasmid interaction along the direction of electric field applied. A pulse with a polarity inducing DNA electrophoresis toward the cells resulted in the 10-fold excess of transfection efficiency compared with a pulse with reverse polarity. Ficoll (10%) which increases medium viscosity or Mg2+ ions (10 mM) which decrease the effective charge of DNA, both reduced transfection efficiency 2-3-fold. These results prove a significant role of DNA electrophoresis in the phenomenon considered. The permeability of cell membranes for an indifferent dye was shown to increase noticeably if the cells were pulsed in the presence of DNA. This indicates a possible interaction of DNA translocated with the pores in an electric field, that results in pore expansion.

Recoverin is a 23-kDa Ca(2+)-binding protein found predominantly in vertebrate photoreceptor cells. Recent electrophysiological and biochemical studies suggest that recoverin may regulate the photoresponse by inhibiting rhodopsin phosphorylation. We find in both cell homogenates and reconstituted systems that the inhibition of rhodopsin phosphorylation by recoverin occurs over a significantly higher free Ca2+ range than previously reported. Half-maximal inhibition occurs at 1.5-3 microM free Ca2+ and is cooperative with a Hill coefficient of approximately 2. Measurements of transducin activation demonstrate that this inhibition prolongs the lifetime of catalytically active rhodopsin. Ca(2+)-recoverin directly inhibits rhodopsin kinase activity, and Ca(2+)-dependent binding of recoverin to rod outer segment membranes is not required for its action. Extrapolation of the in vitro data to in vivo conditions based on simple mass action calculations places the Ca(2+)-recoverin regulation within the physiological free Ca2+ range in intact rod outer segment. The data are consistent with a model in which the fall in free Ca2+ that accompanies rod excitation exerts negative feedback by relieving inhibition of rhodopsin phosphorylation.