Defined subunit arrangement and rab interactions are required for functionality of the HOPS tethering complex.

Within the endomembrane system of eukaryotic cells, multisubunit tethering complexes together with their corresponding Rab-GTPases coordinate vesicle tethering and fusion. Here, we present evidence that two homologous hexameric tethering complexes, the endosomal CORVET (Class C core vacuole/endosome transport) and the vacuolar HOPS (homotypic vacuole fusion and protein sorting) complex, have similar subunit topologies. Both complexes contain two Rab-binding proteins at one end, and the Sec1/Munc18-like Vps33 at the opposite side, suggesting a model on membrane bridging via Rab-GTP and SNARE binding. In agreement, HOPS activity can be reconstituted using purified subcomplexes containing the Rab and Vps33 module, but requires all six subunits for activity. At the center of HOPS and CORVET, the class C proteins Vps11 and Vps18 connect the two parts, and Vps11 binds both HOPS Vps39 and CORVET Vps3 via the same binding site. As HOPS Vps39 is also found at endosomes, our data thus suggest that these tethering complexes follow defined but distinct assembly pathways, and may undergo transition by simple subunit interchange.

Vps41 phosphorylation and the Rab Ypt7 control the targeting of the HOPS complex to endosome-vacuole fusion sites.

Membrane fusion depends on multisubunit tethering factors such as the vacuolar HOPS complex. We previously showed that the vacuolar casein kinase Yck3 regulates vacuole biogenesis via phosphorylation of the HOPS subunit Vps41. Here, we link the identified Vps41 phosphorylation site to HOPS function at the endosome-vacuole fusion site. The nonphosphorylated Vps41 mutant (Vps41 S-A) accumulates together with other HOPS subunits on punctate structures proximal to the vacuole that expand in a class E mutant background and that correspond to in vivo fusion sites. Ultrastructural analysis of this mutant confirmed the presence of tubular endosomal structures close to the vacuole. In contrast, Vps41 with a phosphomimetic mutation (Vps41 S-D) is mislocalized and leads to multilobed vacuoles, indicative of a fusion defect. These two phenotypes can be rescued by overproduction of the vacuolar Rab Ypt7, revealing that both Ypt7 and Yck3-mediated phosphorylation modulate the Vps41 localization to the endosome-vacuole junction. Our data suggest that Vps41 phosphorylation fine-tunes the organization of vacuole fusion sites and provide evidence for a fusion "hot spot" on the vacuole limiting membrane.

Yeast vacuole fusion: a model system for eukaryotic endomembrane dynamics.

Vesicular transport in eukaryotic cells is concluded with the consumption of the vesicle at the target membrane. This fusion process relies on Rabs, tethers and SNAREs. Powerful in vitro fusion systems using isolated organelles were crucial to obtain insights into the underlying mechanism of membrane fusion- from the initiation of fusion to lipid bilayer mixing. Among these systems, yeast vacuoles turned out to be particularly useful as they can be manipulated biochemically and genetically. Studies relying on this organelle have revealed insights into the connection of vacuole fusion to endomembrane biogenesis. A number of fusion factors were identified and characterized over the last several years, and placed into the fusion cascade. Within this review, we will present and discuss the current state of our knowledge on vacuole fusion.

The CORVET tethering complex interacts with the yeast Rab5 homolog Vps21 and is involved in endo-lysosomal biogenesis.

The dynamic equilibrium between vesicle fission and fusion at Golgi, endosome, and vacuole/lysosome is critical for the maintenance of organelle identity. It depends, among others, on Rab GTPases and tethering factors, whose function and regulation are still unclear. We now show that transport among Golgi, endosome, and vacuole is controlled by two homologous tethering complexes, the previously identified HOPS complex at the vacuole and a novel endosomal tethering (CORVET) complex, which interacts with the Rab GTPase Vps21. Both complexes share the four class C Vps proteins: Vps11, Vps16, Vps18, and Vps33. The HOPS complex, in addition, contains Vps41/Vam2 and Vam6, whereas the CORVET complex has the Vps41 homolog Vps8 and the (h)Vam6 homolog Vps3. Strikingly, the CORVET and HOPS complexes can interconvert; we identify two additional intermediate complexes, both consisting of the class C core bound to Vam6-Vps8 or Vps3-Vps41. Our data suggest that modular assembled tethering complexes define organelle biogenesis in the endocytic pathway.

Factor H and atypical hemolytic uremic syndrome: mutations in the C-terminus cause structural changes and defective recognition functions.

Atypical hemolytic uremic syndrome is a disease that is characterized by microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. Mutations in the complement regulator factor H are associated with the inherited form of the disease, and >60% of the mutations are located within the C terminus of factor H. The C-terminus of factor H, represented by short consensus repeat 19 (SCR19) and SCR20, harbors multiple functions; consequently, this study aimed to examine the functional effects of clinically reported mutations in these SCR. Mutant factor H proteins (W1157R, W1183L, V1197A, R1210C, R1215G, and P1226S) were recombinantly expressed and functionally characterized. All six mutant proteins showed severely reduced heparin, C3b, C3d, and endothelial cell binding. By peptide spot analyses, four linear regions that are involved in heparin, C3b, and C3d binding were localized in SCR19 and SCR20. A three-dimensional homology model of the two domains suggests that these four regions form a common binding site across both domains. In addition, this structural model identifies two types of residues: Type A residues are positioned on the SCR surface and are represented by mutants W1157R, W1183L, R1210C, and R1215G; and type B residues are buried within the SCR structure and affect mutations V1197A and P1226S. Mutations of both types of residue result in the same functional defects, namely the reduced binding of factor H to surface-attached C3b molecules and reduced complement regulatory activity at the cell surfaces. The buried type B mutations seem to affect ligand interaction of factor H more severely than the surface-exposed mutations.