In neurons, synaptotagmin1 (Syt1) is thought to mediate the fusion of synaptic vesicles with the plasma membrane when presynaptic Ca 2+ levels rise. However, in vitro reconstitution experiments have failed to recapitulate key characteristics of Ca 2+ -triggered membrane fusion. Using an in vitro single-vesicle fusion assay, we found that membrane-anchored Syt1 enhanced Ca 2+ -sensitivity and fusion speed. This stimulatory activity of membrane-anchored Syt1 dropped as the Ca 2+ level rose beyond physiological levels. Thus, Syt1 requires the membrane anchor to stimulate vesicle fusion at physiological Ca 2+ levels, and may function as a dynamic presynaptic Ca 2+ sensor to control the probability of neurotransmitter release.

Co-immunoprecipitation (co-IP) has become a standard technique, but its protein-band output provides only static, qualitative information about protein–protein interactions. Here we demonstrate a real-time single-molecule co-IP technique that generates real-time videos of individual protein–protein interactions as they occur in unpurified cell extracts. By analysing single Ras–Raf interactions with a 50-ms time resolution, we have observed transient intermediates of the protein–protein interaction and determined all the essential kinetic rates. Using this technique, we have quantified the active fraction of native Ras proteins in xenograft tumours, normal tissue and cancer cell lines. We demonstrate that the oncogenic Ras mutations selectively increase the active-Ras fraction by one order of magnitude, without affecting total Ras levels or single-molecule signalling kinetics. Our approach allows us to probe the previously hidden, dynamic aspects of weak protein–protein interactions. It also suggests a path forward towards precision molecular diagnostics at the protein–protein interaction level.

Coimmunoprecipitation (co-IP) analysis is a useful method for studying protein-protein interactions. It currently involves electrophoresis and western blotting, which are not optimized for detecting weak and transient interactions. In this protocol we describe an advanced version of co-IP analysis that uses real-time, single-molecule fluorescence imaging as its detection scheme. Bait proteins are pulled down onto the imaging plane of a total internal reflection (TIR) microscope. With unpurified cells or tissue extracts kept in reaction chambers, we observe single protein-protein interactions between the surface-immobilized bait and the fluorescent protein-labeled prey proteins in real time. Such direct recording provides an improvement of five orders of magnitude in the time resolution of co-IP analysis. With the single-molecule sensitivity and millisecond time resolution, which distinguish our method from other methods for measuring weak protein-protein interactions, it is possible to quantify the interaction kinetics and active fraction of native, unlabeled bait proteins. Real-time single-molecule co-IP analysis, which takes ∼4 h to complete from lysate preparation to kinetic analysis, provides a general avenue for revealing the rich kinetic picture of target protein-protein interactions, and it can be used, for example, to investigate the molecular lesions that drive individual cancers at the level of protein-protein interactions.

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Clitocybin A is a novel anti-wrinkle cosmetic agent produced by the strain from a Korean native mushroom Clitocybe aurantiaca. In this study, fermentation, extraction, and purification conditions for a large scale production of clitocybin A were optimized, and its cytotoxicity and inhibition activity on the expression of matrix metalloproteinase-1 (MMP-1) were characterized. The mass production of anti-wrinkle agent was achieved according to the 300 L fermentation process with a fedbatch cultivation using the modified yeast-maltose (YM) broth, and a total of 12.5 kg of cell mass was obtained in a 120 L culture broth for 14 days. After extraction and purification, clitocybin A was identified by HPLC. The cytotoxicity of clitocybin A was examined by the MTT assay. When assayed at 100 and 200 µg/ml concentrations, clitocybin A showed no cytotoxicity, demonstrating safety. The inhibition activity of clitocybin A on the expression of MMP-1 was examined against UV irradiation. Oleanolic acid (control group) showed a relatively low MMP-1 inhibiting activity (ca. 16.7%) at 10 µg/ml and showed increased cytotoxicity at higher concentrations. In contrast, clitocybin A showed no cytotoxicity at 100 µg/ml, and exhibited a relatively high MMP-1-inhibiting activity (33.1%). These findings indicate that clitocybin A may be a safe and effective anti-wrinkle agent for use in functional cosmetics.

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