An effector of Ypt6p binds the SNARE Tlg1p and mediates selective fusion of vesicles with late Golgi membranes.

Membrane traffic requires vesicles to fuse with a specific target, and SNARE proteins and Rab/Ypt GTPases contribute to this specificity. In the yeast Saccharomyces cerevisae, the Rab/Ypt GTPase Ypt6p is required for fusion of endosome-derived vesicles with the late Golgi. We have shown previously that activation of Ypt6p depends on its exchange factor, Ric1p-Rgp1p, a peripheral membrane protein complex restricted to the Golgi. We show here that a conserved trimeric protein complex, VFT (Vps52/53/54), binds directly to Ypt6p:GTP. Localization of VFT to the Golgi requires Ypt6p, but is unaffected in gos1 and tlg1 mutants, in which late Golgi integral membrane proteins, including SNAREs, are mislocalized. The VFT complex also binds directly to the N-terminal domain of the SNARE Tlg1p, both in vitro and in vivo, in a Ypt6p-independent manner. We suggest that the VFT complex links vesicles containing Tlg1p to their target, which is defined by the local activation of Ypt6p.     

Mutational analysis of the human KDEL receptor: distinct structural requirements for Golgi retention, ligand binding and retrograde transport.

The KDEL receptor is a seven-transmembrane-domain protein that is responsible for the retrieval of endoplasmic reticulum (ER) proteins from the Golgi complex. It is a temporary resident of the Golgi apparatus: upon binding a KDEL-containing ligand, it moves to the ER, where the ligand is released. We have expressed mutant forms of the human receptor in COS cells and examined their intracellular locations and ligand-binding capacities. We show that ligand binding is dependent on charged residues within the transmembrane domains. Surprisingly, retrograde transport of occupied receptor is unaffected by most mutations in the cytoplasmic loops, but is critically dependent upon an aspartic acid residue in the seventh transmembrane domain. Retention in the Golgi apparatus requires neither ligand binding nor this aspartate residue, and thus is independent of receptor recycling. We suggest that movement of the receptor is controlled by conformational changes and intermolecular interactions within the membrane bilayer.     

Hsp70 accelerates the recovery of nucleolar morphology after heat shock.

The major heat-shock protein, hsp70, is synthesized by cells of many organisms in response to stress. In the present study, Drosophila hsp70 was expressed from cloned genes in mouse L cells and monkey COS cells and detected by immunofluorescence using monoclonal antibodies. Hsp70 is found mostly but not exclusively in the nucleus of unstressed cells. For several hours after a short heat shock, however, it is strongly concentrated in nucleoli. Nucleoli are transiently damaged by such a heat shock: their morphology changes and assembly and export of ribosomes is blocked for several hours. This block can be visualized by addition of actinomycin D: under normal conditions pre-ribosomes are chased out of nucleoli, and the latter shrink dramatically, but no such shrinking is seen in heat-shocked cells. High levels of hsp70 can be produced in unstressed COS cells by transfecting them with an appropriate expression plasmid. Such cells show a more rapid recovery of nucleolar morphology following a heat shock than do untransfected cells. Furthermore, heat shock does not prevent shrinkage of their nucleoli in the presence of actinomycin, which indicates that ribosome export also recovers rapidly when pre-synthesized hsp70 is present. I suggest that an important function of hsp70 is to catalyze reassembly of damaged pre-ribosomes and other RNPs after heat shock.     

Bsd2 binds the ubiquitin ligase Rsp5 and mediates the ubiquitination of transmembrane proteins

Membrane proteins destined for the vacuolar or lysosomal lumen are typically ubiquitinated, the ubiquitin serving as a targeting signal for the multivesicular body pathway. The RING-domain ubiquitin ligase Tul1 is an integral membrane protein that modifies the yeast vacuolar enzyme carboxypeptidase S (Cps1), the polyphosphatase Ppn1/Phm5 and other proteins containing exposed hydrophilic residues within their transmembrane domains (TMDs). Here we show that Bsd2 provides an alternative ubiquitination mechanism for Cps1, Phm5 and other proteins. Bsd2 is a three-TMD protein with a PPXY motif that binds the HECT domain ubiquitin ligase Rsp5. It can thus act as a specific adaptor linking Rsp5 to its substrates. Like Tul1, the Bsd2 system recognises polar TMDs. Bsd2 also controls the vacuolar targeting of a manganese transporter and a mutant plasma membrane ATPase, and together with the ER retrieval receptor Rer1, it protects cells from stress. We suggest that Bsd2 has a wide role in the quality control of membrane proteins. Bsd2 is the yeast homologue of human NEDD4 binding protein N4WBP5, which may therefore have similar functions.