No abstract

Nachweis in der Zelle: Mikrokügelchen (2 μm), die kovalent mit Calciumsensoren beladen sind, können effizient in lebende Zellen eingebracht werden, um die intrazellulären Änderungen der Ca2+‐Konzentration in Echtzeit zu analysieren. Das Bild zeigt die ratiometrische Fluoreszenzanalyse (Verhältnis 400/475 nm) der Freisetzung von Ca2+‐Ionen in mit den Mikrokügelchen beladenen Zellen in Echtzeit.

One of the most important actions of insulin is the stimulation of the uptake of glucose into fat and muscle cells. Crucial to this response is the translocation of GLUT4 (glucose transporter-4) to the plasma membrane. The insulin-stimulated GLUT4 vesicle docking at the plasma membrane requires an interaction between VAMP-2 (vesicle-associated membrane protein-2) on the GLUT4 vesicle and syntaxin-4 in the plasma membrane. In the basal state, munc18c is thought to preclude GLUT4 vesicle docking by inhibiting this interaction. Here, we have used FCS (fluorescence correlation spectroscopy) in single living cells to show that munc18c binds to syntaxin-4 in both the basal and insulin-stimulated states. We show that munc18c contains two binding sites for syntaxin-4, one of which is disrupted by insulin, while the other is activated by insulin. Insulin-triggered repositioning of munc18c on syntaxin-4 in this way in turn allows syntaxin-4 to adopt its 'open' conformation and bind VAMP-2, resulting in the docking of the GLUT4 vesicle at the cell surface. The results also demonstrate the utility of using FCS in intact single living cells to elucidate cell signalling events.

Insulin triggers the translocation of glucose transporter GLUT4 to the plasma membrane. To understand the nature of the missing links between upstream insulin activated kinases and proteins of the GLUT4 translocation apparatus, the role of 80K-H was examined to test if it was one such missing link in live cells. Fluorescence correlation spectroscopy showed that the mobility of 80K-H was significantly decreased by insulin stimulation. This was dependent on the presence of PKCzeta and an intact binding site for PKCzeta. Insulin also increased the mobility of munc18c in an 80K-H- and PKCzeta dependent manner. These results indicate that insulin induces dynamic associations between PKCzeta, 80K-H, and munc18c and that 80K-H may act as a key signaling link between PKCzeta and munc18c in live cells.