In vitro fusion between Saccharomyces cerevisiae secretory vesicles and cytoplasmic-side-out plasma membrane vesicles

Role of Cholesterol, DOTAP, and DPPC in Prostasome/Spermatozoa Interaction and Fusion

Prostasomes are membranous vesicles present in ejaculated human semen. They are very rich in cholesterol and can interact with spermatozoa. Their physiological roles are still under study. Prostasomes were mixed with liposomes prepared from various lipids, such as N-[1-(2,3-dioleoyloxy)propyl]-N,N,N-trimethylammonium (DOTAP), DOTAP/1,2-dipalmytoyl-sn-glycero-3-phosphorylcholine (DPPC, 4:1 molar ratio) and DOTAP/cholesterol (4:1, molar ratio) at different pH values (5-8). The mixing of the lipid phases (fusion) was determined by the relief of octadecyl rhodamine B chloride (R(18)) self-quenching and the radii of the vesicles, by light scattering measurements. The mixing of lipids and the radii of prostasomes were both influenced by the addition of liposome, although in a different manner. The ability of prostasomes (modified by previous treatment with liposomes) to transfer lipid to spermatozoa was also measured. Pretreatment with DOTAP decreased the phenomenon and addition of DPPC abolished it. On the other hand, pretreatment of prostasomes with DOTAP/cholesterol liposomes did not affect the transfer of lipid between prostasome and spermatozoa. Therefore, the ability of vesicles to fuse (or, at least, to exchange the lipid component) was affected by the enrichment in either natural or artificial lipid. This may open new possibilities for the modulation of spermatozoa capacitation and acrosome reaction.

Novel lipid transfer property of two mitochondrial proteins that bridge the inner and outer membranes

This study provides evidence of a novel function for mitochondrial creatine kinase (MtCK) and nucleoside diphosphate kinase (NDPK-D). Both are basic peripheral membrane proteins with symmetrical homo-oligomeric structure, which in the case of MtCK was already shown to allow crossbridging of lipid bilayers. Here, different lipid dilution assays clearly demonstrate that both kinases also facilitate lipid transfer from one bilayer to another. Lipid transfer occurs between liposomes mimicking the lipid composition of mitochondrial contact sites, containing 30 mol % cardiolipin, but transfer does not occur when cardiolipin is replaced by phosphatidylglycerol. Ubiquitous MtCK, but not NDPK-D, shows some specificity in the nature of the lipids transferred and it is not active with phosphatidylcholine alone. MtCK can undergo reversible oligomerization between dimeric and octameric forms, but only the octamer can bridge membranes and promote lipid transfer. Cytochrome c, another basic mitochondrial protein known to bind to anionic membranes but not crosslinking them, is also incapable of promoting lipid transfer. The lipid transfer process does not involve vesicle fusion or loss of the internal contents of the liposomes.

Fusion of synaptic vesicles and plasma membrane in the presence of synaptosomal soluble proteins

Fusion between synaptic vesicles and plasma membranes isolated from rat brain synaptosomes is regarded as a model of neurosecretion. The main aim of current study is to investigate whether the synaptosomal soluble proteins are essential members of Ca(2+)-triggered fusion examined in this system. Fusion experiments were performed using fluorescent dye octadecylrhodamine B, which was incorporated into synaptic vesicle membranes at self-quenching concentration. The fusion of synaptic vesicles, containing marker octadecylrhodamine B, with plasma membranes was detected by dequenching of the probe fluorescence. Membrane fusion was not found in Ca(2+)-supplemented buffer solution, but was initiated by the addition of the synaptosomal soluble proteins. When soluble proteins were treated with trypsin, they lost completely the fusion activity. These experiments confirmed that soluble proteins of synaptosomes are sensitive to Ca(2+) signal and essential for membrane fusion. The experiments, in which members of fusion process were treated with monoclonal antibodies raised against synaptotagmin and synaptobrevin, have shown that antibodies only partially inhibited fusion of synaptic vesicles and plasma membranes in vitro. These results indicate that other additional component(s), which may or may not be related to synaptobrevin or synaptotagmin, mediate this process. It can be assumed that fusion of synaptic vesicles with plasma membranes in vitro depends upon the complex interaction of a large number of protein factors.

Fusion of lamellar body with plasma membrane is driven by the dual action of annexin II tetramer and arachidonic acid

Annexin II has been implicated in membrane fusion during the exocytosis of lamellar bodies from alveolar epithelial type II cells. Most previous studies were based on the fusion assays by using model membranes. In the present study, we investigated annexin II-mediated membrane fusion by using isolated lamellar bodies and plasma membrane as determined by the relief of octadecyl rhodamine B (R18) self-quenching. Immunodepletion of annexin II from type II cell cytosol reduced its fusion activity. Purified annexin II tetramer (AIIt) induced the fusion of lamellar bodies with the plasma membrane in a dose-dependent manner. This fusion is Ca2+-dependent and is highly specific to AIIt because other annexins (I and II monomer, III, IV, V, and VI) were unable to induce the fusion. Modification of the different functional residues of AIIt by N-ethylmaleimide, nitric oxide, or peroxynitrite abolished AIIt-mediated fusion. Arachidonic acid enhanced AIIt-mediated fusion and reduced its Ca2+ requirement to an intracellularly achievable level. This effect is due to membrane-bound arachidonic acid, not free arachidonic acid. Other fatty acids including linolenic acid, palmitoleic acid, myristoleic acid, stearic acid, palmitic acid, and myristic acid had little effect. AIIt-mediated fusion was suppressed by the removal of arachidonic acid from lamellar body and plasma membrane using bovine serum albumin. The addition of arachidonic acid back to the arachidonic acid-depleted membranes restored its fusion activity. Our results suggest that the fusion between lamellar bodies with the plasma membrane is driven by the synergistic action of AIIt and arachidonic acid.