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Vesicle tethering complexes in membrane traffic

MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK

View larger version (9K): [in a new window]   Fig. 1. Steps in the delivery of vesicles to the correct organelle. (1) An intracellular transport vesicle approaches its destination organelle either by diffusion or motor-mediated directed transport. (2) The vesicle is then proposed to be tethered to the organelle by protein complexes and long coiled-coil proteins. (3) A v-SNARE protein on the vesicle then engages a t-SNARE on the target, forming a four-helical bundle whose assembly drives the two bilayers into close proximity, (4) thereby causing membrane fusion. Both vesicle tethering and SNARE assembly have been referred to by others as `docking', so to avoid confusion we use only the former terms here.

View larger version (30K): [in a new window]   Fig. 2. Putative tethering complexes in the yeast secretory pathway. Protein complexes that have been found to have a role in particular vesicular transport steps are indicated next to those steps. The role of early and late endosomes in yeast is contentious, and so for simplicity this compartment has been shown as a single organelle.

View larger version (32K): [in a new window]   Fig. 3. Composition of proposed tethering complexes. For each complex the known components in the yeast S. cerevisiae are shown, arranged by size, and identifiable domains indicated. In each case the standard gene name in the Saccharomyces Genome Database is given first, followed by alternative names that have also been used in recent publications. Vps51p is encoded by the open reading frame YKR020w (Elizabeth Conibear, personal communication). The two sets of related subunits of the TRAPP complexes are indicated by different colours. Homologues of most of these proteins exist in higher eukaryotes, but in some cases have extra domains. Thus in mammals Sec5 has an N-terminal TIG domain, Exo84 a PH domain, Vam6 a CNH domain (Caplan et al., 2001) and Vam2 a C-terminal RING-H2 domain (Radisky et al., 1997). Vps54 has an N-terminal zinc-finger-like domain in Drosophila and C. elegans, but not in mammals. Vam6 in both yeast and higher eukaryotes has a conserved half RING domain (C2HC) at its C-terminus. The `p' has been removed from the yeast protein names for clarity.

View larger version (38K): [in a new window]   Fig. 4. The presence of a shared domain in components of the COG complex and the exocyst. (A) Sequence alignment of the N-terminal amphipathic helical regions of the indicated components of the human COG complex and exocyst. Residues are shaded if identical (black) or conserved (grey) in at least three proteins. Grey bars show the regions predicted to form coiled-coil (the hydrophobic heptad repeat indicated by black circles). (B) Prediction of the propensity of the subunits of the human COG complex and human exocyst to form coiled coils. The length of the x-axis corresponds to the length of the proteins, with residue numbers indicated at the bottom of the figure. On the y-axis is plotted the probability of a coiled-coil being at each residue of the protein, as determined by the algorithm of Lupas (Lupas, 1996) using the MacStripe program (v2.0b1) with a window length of 28 residues, the MTIDK matrix and weighting of hydrophobic residues. Red bars indicate regions that are aligned in (A). Blue bars indicate longer regions of sequence similarity between Cog3, Cog6 and Exo70 [see alignment in Whyte and Munro (Whyte and Munro, 2001)].