Christien Merrifield | PubMed Health

PubMed2005

The Wnt signalling effector Dishevelled forms dynamic protein assemblies rather than stable associations with cytoplasmic vesicles.

Dishevelled is a crucial effector upstream in the Wnt signalling pathway, but the molecular mechanism by which it transduces the Wnt signal remains elusive. Dishevelled is a cytoplasmic protein with a strong tendency to form puncta, which correlates with its potent activity in stimulating Wnt signal transduction when overexpressed. These puncta are thought to reflect cytoplasmic vesicles. However, we show here that the mammalian Dishevelled protein Dvl2 does not colocalise with known vesicle markers for clathrin-mediated or clathrin-independent endocytic pathways. Furthermore, Dvl2 puncta do not stain with lipid dyes, indicating that these puncta do not contain membranes. Instead, our evidence from live imaging by TIRF microscopy of Dvl2 tagged with green fluorescent protein (GFP-Dvl2) revealed that these puncta move in and out of the evanescent field near the plasma membrane in an undirected fashion, and that they can grow by collision and fusion. Furthermore, high-resolution confocal microscopy and photobleaching experiments indicate that the GFP-Dvl2 puncta are protein assemblies; there is a constant exchange of GFP-Dvl2 between puncta and a diffuse cytoplasmic pool, which, therefore, are in a dynamic equilibrium with each other. The same is true for the DIX domain of Dvl2 itself and also for Axin-GFP, which equilibrates between the punctate and cytosolic pools. Our evidence indicates that Dvl2 and Axin puncta are dynamic protein assemblies rather than cytoplasmic vesicles.

Adaptor Proteins, Signal Transducing↗

PubMed2004

Visualizing synaptic ribbons in the living cell.

Visual and auditory information is encoded by sensory neurons that tonically release neurotransmitter at high rates. The synaptic ribbon is an essential organelle in nerve terminals of these neurons. Its precise function is unknown, but if the ribbon could be visualized in a living terminal, both its own dynamics and its relation to calcium and vesicle dynamics could be studied. We designed a short fluorescent peptide with affinity for a known binding domain of RIBEYE, a protein unique to the ribbon. When introduced via a whole-cell patch pipette, the peptide labeled structures at the presynaptic plasma membrane of ribbon-type terminals. The fluorescent spots match in size, location, number, and distribution the known features of synaptic ribbons. Furthermore, fluorescent spots mapped by confocal microscopy directly match the ribbons identified by electron microscopy in the same cell. Clearly the peptide binds to the synaptic ribbon, but even at saturating concentrations it affects neither the morphology of the ribbon nor its tethering of synaptic vesicles. It also does not inhibit exocytosis. Using the peptide label, we observed that the ribbon is immobile over minutes and that calcium influx is concentrated at the ribbon. Finally, we find that each ribbon in a retinal bipolar cell contains approximately 4000 molecules of RIBEYE, indicating that it is the major component of the synaptic ribbon.

Amino Acid Motifs↗