Vesicle Transport: a Close Dispatch Collaboration of Rabs and Effectors

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Vesicle Transport: A Close Dispatch Collaboration of Rabs and Effectors

Anne Spang biogenesis in yeast. They found that either treatment with antibodies against Yip1p or mutation of YIP1 blocked COPII vesicle generation at the ER membrane. COPII vesicles transport proteins destined for Interfering with Yip1p function thus has a greater effect secretion from the ER to the Golgi apparatus. A recent on COPII budding than does blocking Ypt1p. This indi- study has shown that, in budding yeast, the formation cates that Yip1p must do more than just interact with of COPII vesicles requires Yip1p, an effector protein of Ypt1p. Yip1p is an integral membrane protein that a Rab GTPase. cycles between the ER and the Golgi apparatus, and so it is also a passenger in COPII vesicles. It forms a het- eromeric complex with the related protein Yif1p, which In eukaryotic cells, proteins destined for secretion are can also interact with Ypt1p [11]. Yeast cells contain translocated into the endoplasmic reticulum (ER) and soluble, as well as membrane-bound, pools of Uso1p packaged there into so-called COPII vesicles. In and Ypt1p [12], and can be recruited to ER or Golgi budding yeast, COPII vesicles are bound for the Golgi membranes, given that these compartments have inter- apparatus, where they are captured and tethered before acting molecules. Furthermore, Yip1p has been shown fusion. A specific cytosolic protein coat is required for to interact genetically with Uso1p and Ypt1p, but not vesicle formation from a donor compartment such as with other Golgi-localized proteins. the ER. Similarly, the fusion of the vesicles with their What role might Yip1p have in the biogenesis of target membrane involves a collaboration of specific COPII vesicles? One immediate possibility invokes the components such as Rab GTPases, tethering factors connection with Ypt1p. Sivars et al. [13] reported and ‘SNARE’ complexes. This is well known and has evidence that a mammalian Yip family member, Yip3, is become doctrine; but typically for a biological process, a guanine dissociation inhibitor (GDI) displacement things are turning out to be more complicated than at factor for Rabs of the endosomal pathway, Rab9 in par- first appeared to be the case. ticular; thus Yip3 delivers Rab9 to the membrane. Yip1p Genetic and biochemical analysis have led to a might play a similar role at the ER, though blocking Ypt1 relatively detailed model for the mechanisms of COPII function does not prevent COPII vesicle formation. It vesicle formation, docking and fusion. The coat should be noted, however, that while Yip3 and Yip1p components contain all the activities required to bud are both members of the Yip family, their sequence sim- COPII coated vesicles from the donor ER membrane, ilarity is relatively low. Heidtman et al. [10] suggest that including modules that mediate cargo recruitment. Yip1p’s role in COPII vesicle biogenesis might actually Sec24p and its homologues are capable of recognizing be independent from its ability to interact with Ypt1p. specific motifs that lead to trapping and concentration We still do not know whether vesicles are formed con- of SNAREs and cargo proteins to the polymerizing coat tinuously inside cells or if they are formed on demand, [1–4]. Escort proteins such as Shr3p, Chs7p and with their cargo level depending on how much protein Vma22p have also been identified which facilitate the needs to be exported at any particular time. At the Golgi exit of cargo from the ER via COPII vesicles [5,6]. apparatus, at least, it does seem as though the amount But recent studies indicate that we are still lacking of cargo influences vesicle flow [14], either directly or some key parts of the picture. Morsomme and Riezman indirectly. A possible role for Yip1p would be to limit dif- [7] found that Uso1p, the tethering factor that captures fusion: it might somehow sense and keep together the COPII vesicles at the Golgi apparatus, and Ypt1p, the cargo molecules, and thus allow a vesicle to be built. Rab GTPase at the cis-Golgi membrane, are required Uso1p and Ypt1p may select and recruit specific cargo for inclusion of GPI-anchored proteins into COPII vesi- molecules, while Yip1p clusters cargoes, increasing the cles at the ER, but are not essential for the uptake of affinity for the coat and the likelihood for vesicle forma- other cargoes into the emerging bud. They went on to tion at a specific point. Consistent with this view, muta- show that mutant cells with defects of the conserved tions of Yip1p are synthetically lethal with those of oligomeric Golgi (COG) complex, but not the transport Sec12p, the guanine nucleotide exchange factor for the protein particle (TRAPP) complex, are defective in small GTPase Sar1p, which is essential for the polymer- protein sorting at the ER. The COG and TRAPP com- ization of the COPII coat [15]. plexes are required for efficient vesicle consumption at Proteins are often required to perform multiple tasks the Golgi apparatus [8,9]. It is surprising that Golgi- in cells, specificity coming from their associations with localized proteins can influence cargo uptake at the ER. different protein complexes. This may be true of Yip1p. Complicating matters further, Barlowe and col- In one possible scenario, a Yif1p–Yip1p complex acts leagues [10] have recently reported that the Ypt1p- as cargo barrier or impediment to diffusion. Once interacting protein Yip1p is required for COPII vesicle enough cargo has assembled — as assessed by the incoming coat proteins, which polymerize and stabilize Friedrich Miescher Laboratory of the Max Planck Society, cargo assembly — the diffusion barrier is no longer Spemannstrasse 39, D-72076 Tuebingen, Germany. needed. Yop1p, another protein that can interact with E-mail: [email protected] Yip1p, might then come into play, severing the diffusion Dispatch R34

limitation set by Yif1p–Yip1p. The possibility that Yop1p is a negative regulator of Yip1p function in vesicle biogenesis is suggested by the observation that YOP1 overexpression blocks transport of the cargo protein carboxypeptidase Y to the Golgi apparatus, and this block can be rescued by co-overexpression of YIP1 [16]. The vesicle forms, carrying some Yif1p–Yip1p complexes which may also function at later steps in the secretory pathway and be retrieved from the trans- Golgi compartment. And finally, uncomplexed Yip1p might act as a GDI displacement factor for Ypt1p, though whether Yip1p really is the GDI displacement factor for Ypt1p remains to be demonstrated. Thus, Yip1p function might be determined, not only by localization to different compartments, but also by the hetero-oligomeric state of the protein. Purified coat components and guanine nucleotides are in principle sufficient to drive vesicle formation, but a cell has to ensure that the budding event is regulated in an appropriate spatiotemporal manner. We are only beginning to grasp how this is done.

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