Sequential transport from early to late endosomes requires the coordinated activities of the small GTPases Rab5 and Rab7. The transition between early and late endosomes could be mediated either through transport carriers or by Rab conversion, a process in which the loss of Rab5 from an endosome occurs concomitantly to the acquisition of Rab7. We demonstrate that Rab conversion is the mechanism by which proteins pass from early to late endosomes in Caenorhabditis elegans coelomocytes. Moreover, we identified SAND-1/Mon1 as the critical switch for Rab conversion in metazoa. SAND-1 serves a dual role in this process. First, it interrupts the positive feedback loop of RAB-5 activation by displacing RABX-5 from endosomal membranes; second, it times the recruitment of RAB-7, probably through interaction with the HOPS complex to the same membranes. SAND-1/Mon1 thus acts as a switch by controlling the localization of RAB-5 and RAB-7 GEFs.

In Saccharomyces cerevisiae, the synthesis of chitin is temporally and spatially regulated through the transport of Chs3p (chitin synthase III) to the plasma membrane in the bud neck region. Traffic of Chs3p from the trans‐Golgi network (TGN)/early endosome to the plasma membrane requires the function of Chs5p and Chs6p. Chs6p belongs to a family of four proteins that we have named ChAPs for Chs5p‐Arf1p‐binding Proteins. Here, we show that all ChAPs physically interact not only with Chs5p but also with the small GTPase Arf1p. A short sequence at the C‐terminus of the ChAPs is required for protein function and the ability to bind to Chs5p. Simultaneous disruption of two members, Δbud7 and Δbch1, phenocopies a Δchs6 or Δchs5 deletion with respect to Chs3p transport. Moreover, the ChAPs interact with each other and can form complexes. In addition, they are all at least partially localized to the TGN in a Chs5p‐dependent manner. Most importantly, several ChAPs can interact physically with Chs3p. We propose that the ChAPs facilitate export of cargo out of the Golgi.

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Abstract. Tub4p is a novel tubulin in Saccharomyces cerevisiae that most closely resembles ~-tubulin. We report in this manuscript that the essential Tub4p is associated with the inner and outer plaques of the yeast microtubule organizing center, the spindle pole body (SPB). These SPB substructures are involved in the attachment of the nuclear and cytoplasmic microtubules, respectively (Byers, B., and L. Goetsch. 1975. J. T HE number, direction, and polarity of microtubules are organized by organelles called microtubule organizing centers (MTOC) 1. In Saccharomyces cerevisiae, microtubule organizing functions are provided by the spindle pole body (SPB) (see Fig. 8 A). The SPB is a cylindrical multilaminated structure that is embedded in the nuclear envelope. SPB substructures are detectable by EM (Byers, 1981a,b;Byers and Goetsch, 1975). The central plaque serves to anchor the SPB in the nuclear envelope. The inner and outer plaques nucleate the nuclear and cytoplasmic microtubules, respectively. An additional substructure of the SPB, the half bridge, is an extension of the central plaque along the cytoplasmic margin of the nuclear envelope. The half bridge has important functions in SPB duplication.The SPB, in common with centrosomes of higher eukaryotes, shows cell cycle-dependent behavior (Byers and Goetsch, 1975). In G1 of the cell cycle, the single SPB in each yeast cell is duplicated. The duplicated SPBs undergo separation to form the poles of the spindle. Motor proteins and microtubules are required for SPB separation and Address all correspondence to Elmar Schiebel,