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F S Cohen

Publications and source records attributed to F S Cohen.

83 records · Page 5Linked to original sources
PubMed1984

Separation of the osmotically driven fusion event from vesicle-planar membrane attachment in a model system for exocytosis.

We demonstrate that there are two experimentally distinguishable steps in the fusion of phospholipid vesicles with planar bilayer membranes. In the first step, the vesicles form a stable, tightly bound pre-fusion state with the planar membrane; divalent cations (Ca++) are required for the formation of this state if the vesicular and/or planar membrane contain negatively charged lipids. In the second step, the actual fusion of vesicular and planar membranes occurs. The driving force for this step is the osmotic swelling of vesicles attached (in the pre-fusion state) to the planar membrane. We suggest that osmotic swelling of vesicles may also be crucial for biological fusion and exocytosis.

Calcium↗
PubMed1983

Comparison of the macroscopic and single channel conductance properties of colicin E1 and its COOH-terminal tryptic peptide.

A COOH-terminal tryptic fragment (Mr approximately equal to 20,000) of colicin E1 has been proposed to contain the membrane channel-forming domain of the colicin molecule. A comparison is made of the conductance properties of colicin E1 and its COOH-terminal fragment in planar bilayer membranes. The macroscopic and single channel properties of colicin E1 and its COOH-terminal tryptic fragment are very similar, if not indistinguishable, implying that the NH2-terminal, two-thirds of the colicin E1 molecule, does not significantly influence its channel properties. The channel-forming activity of both polypeptides is dependent upon the presence of a membrane potential, negative on the trans side of the membrane. The average single channel conductance of colicin E1 and the COOH-terminal fragment is 20.9 +/- 3.9 and 19.1 +/- 2.9 picosiemens, respectively. The rate at which both proteins form conducting channels increases as the pH is lowered from 7 to 5. Both molecules require negatively charged lipids for activity to be expressed, exhibit the same ion selectivity, and rectify the current to the same extent. Both polypeptides associate irreversibly with the membrane in the absence of voltage, but subsequent formation of conducting channels requires a negative membrane potential.

Amino Acid Sequence↗
PubMed1982

Osmotic swelling of phospholipid vesicles causes them to fuse with a planar phospholipid bilayer membrane.

Fusion of phospholipid vesicles with planar bilayer membranes occurs if the vesicles that contact the planar membrane swell osmotically after the replacement in their medium of an impermeant solute by a permeant one. This finding directly demonstrates that osmotic swelling is a driving force for vesicle-planar membrane fusion. The method used to induce vesicle swelling and fusion may have relevance for biological systems.

Calcium↗
PubMed1982

A study of the relationship between child abuse and drug addiction in 178 patients: preliminary results.

Social histories and demographic data were collected on 178 patients--101 in the United States and 77 in Australia--in treatment for drug/alcohol addiction. The purpose of the study was to determine the degree to which a correlation exists between child abuse/neglect and the later onset of drug/alcohol addiction patterns in the abuse victims. The questionnaire explored such issues as family intactness, parental violence/abuse/neglect, parental drug abuse, sibling relationships and personal physical/sexual abuse histories, including incest and rape. The study determined that 84% of the sample reported a history of child abuse/neglect.

Adult↗
PubMed1980

Micromolar Ca2+ stimulates fusion of lipid vesicles with planar bilayers containing a calcium-binding protein.

Fusion of phospholipid vesicles with a planar phospholipid bilayer membrane that contains a calcium-binding protein appears to mimic the essential aspects of cytoplasmic-vesicle fusion with plasma membranes (exocytosis) in that (i) there is a low basal rate of fusion in the absence of Ca2+, (ii) this basal rate is enormously increased by micromolar (approximately 10 microM) amounts of Ca2+, and (iii) this rate is not increased by millimolar Mg2+. Essential to this process is an osmotic gradient across the planar membrane, with the side containing the vesicles hyperosmotic to the opposite side. Similar osmotic gradients or their equivalent may be crucial for biological fusion events.

Calcium↗
PubMed1980

Fusion of phospholipid vesicles with planar phospholipid bilayer membranes. I. Discharge of vesicular contents across the planar membrane.

Multilamellar phospholipid vesicles are introduced into the cis compartment on one side of a planar phospholipid bilayer membrane. The vesicles contain a water-soluble fluorescent dye trapped in the aqueous phases between the lamellae. If a vesicle containing n lamellae fuses with a planar membrane, an n-1 lamellar vesicle should be discharged into the opposite trans compartment, where it would appear as a discernible fluorescent particle. Thus, fusion events can be assayed by counting the number of fluorescent particles appearing in the trans compartment. In the absence of divalent cation, fusion does not occur, even after vesicles have been in the cis compartment for 40 min. When CaCl2 is introduced into the cis compartment to a concentration of greater than or equal to 20 mM, fusion occurs within the next 20 min; it generally ceases thereafter because of vesicle aggregation in the cis compartment. With approximately 3 x 10(8) vesicles/cm3 in the cis compartment, about 25-50 fusion events occur following CaCl2 addition. The discharge of vesicular contents across the planar membrane is the most convincing evidence of vesicle-membrane fusion and serves as a model for that ubiquitous biological phenomenon--exocytosis.

Exocytosis↗
PubMed1980

Fusion of phospholipid vesicles with planar phospholipid bilayer membranes. II. Incorporation of a vesicular membrane marker into the planar membrane.

Fusion of multilamellar phospholipid vesicles with planar phospholipid bilayer membranes was monitored by the rate of appearance in the planar membrane of an intrinsic membrane protein present in the vesicle membranes. An essential requirement for fusion is an osmotic gradient across the planar membrane, with the cis side (the side containing the vesicles) hyperosmotic to the opposite (trans) side; for substantial fusion rates, divalent cation must also be present on the cis side. Thus, the low fusion rates obtained with 100 mM excess glucose in the cis compartment are enhanced orders of magnitude by the addition of 5-10 mM CaCl2 to the cis compartment. Conversely, the rapid fusion rates induced by 40 mM CaCl2 in the cis compartment are completely suppressed when the osmotic gradient (created by the 40 mM CaCl2) is abolished by addition of an equivalent amount of either CaCl2, NaCl, urea, or glucose to the trans compartment. We propose that fusion occurs by the osmotic swelling of vesicles in contact with the planar membrane, with subsequent rupture of the vesicular and planar membranes in the region of contact. Divalent cations catalyze this process by increasing the frequency and duration of vesicle-planar membrane contact. We argue that essentially this same osmotic mechanism drives biological fusion processes, such as exocytosis. Our fusion procedure provides a general method for incorporating and reconstituting transport proteins into planar phospholipid bilayer membranes.

Calcium Chloride↗
PubMed1977

The kinetic mechanism of action of an uncoupler of oxidative phosphorylation.

The chemiosmotic hypothesis predicts that the mechanism by which weak acids uncouple oxidative phosphorylation in mitochondria is identical to the mechanism by which they transport hydrogen ions across artificial bilayer membranes. We report here the results of a kinetic study of uncoupler-mediated hydrogen ion transport across bilayer membranes. We made electrical relaxation measurements on black lipid membranes exposed to the substituted benzimidazole 5,6-dichloro-2-trifluoromethylbenzimidazole. The simplest model consistent with our experimental data allowed us to deduce values for adsorption coefficients and rate constants. Our major conclusions are that the back diffusion of the neutral species is the rate limiting step for the steady state transport of hydrogen ions, that both the neutral and charged forms of the uncoupler adsorb strongly to the interfaces, and that the reactions at the membrane-solution interfaces occur sufficiently rapidly for equilibrium to be maintained. Independent measurements of the adsorption coefficients of both the neutral and anionic forms of the weak acid and also of the permeability of the membrane to the neutral form agreed well with the values deduced from the kinetic study.

Benzimidazoles↗