Structural insights into assembly of TRAPPII and its activation of Rab11/Ypt32

The nobel prize in physiology or medicine 2013 was awarded jointly to three scientists for their discoveries of machinery regulating vesicle traffic, a major transport system in our cells. Cells produce molecules such as hormones, neurotransmitters, cytokines, and enzymes that have to be delivered to other places inside the cell, or exported out of the cell, at exactly the right moment and position to keep the proper function of the cells. These molecules were known to be packaged into vesicles, and their transportation in the cell is driven primarily by vesicle-mediated membrane trafficking in the secretory and endocytic pathways [1]. Vesicle-mediated membrane trafficking is a highly sequential process including vesicle formation, vesicle translocation to, vesicle tethering on and vesicle fusion with the target compartment [2]. Among them, tethering is the initial interaction between a vesicle and its target membrane and thus plays a critical role to guide vesicles to specific membranes [2]. It is highly regulated by tethering factors together with Rab/Ypt guanosine triphosphatases (GTPases) and their diverse downstream effectors [3,4]. The roles of tethering factors involve acting as linkers between different target membrane and transport vesicles through specifically binding the coat proteins [5,6], being effectors or guanine nucleotide exchange factors (GEFs) for Rab/Ypt GTPases [4], and promoting the organization of the SNARE proteins involved in the vesicle fusion [7]. Almost all tethering factors fall into two categories: the long coiled-coil proteins and the multisubunit tethering complexes (MTCs) [8]. The former includes p115, Uso1, GM130, etc. [9] and MTCs comprise HOPS, CORVET, exocyst, COG, GARP, Dsl1 and TRAPP complexes [2]. They exist on a variety of compartments and contribute to vesicle fusion specificity.

The TRAPP (transport protein particle) complexes, originally identified from yeast in 1998, are highly conserved across all known eukaryotes [10, 11, 12, 13]. In most species studied to date, including Saccharomyces cerevisiae, Aspergillus nidulans, Drosophila, and human being, there are two forms, TRAPPII and TRAPPIII, with TRAPPI now thought to appear only in vitro and result from dissociation of other two TRAPPs either in vivo or during extract preparation [14, 15, 16, 17∗]. TRAPP complexes act as GEFs for Rab GTPases to convert them from the inactive GDP-bound form to the active GTP-bound form. Specifically, TRAPPIII activates Rab1, while TRAPPII primarily activates Rab11 and also has some activity on Rab1, based on genetic, biochemical and cellular studies [18, 19, 20, 21, 22∗∗, 23∗, 24∗]. Thus, they function in the endoplasmic reticulum to cis-Golgi transport, the maturation of Golgi cisternae, the endosome-to-Golgi transport, and the biogenesis of autophagosomes [5,6,10,13,21,25,26]. Consistent with the TRAPP complexes acting in key membrane trafficking pathways, variations in the genes encoding their subunits result in a spectrum of human diseases including neurodevelopmental disorders, muscular dystrophies and skeletal dysplasias [27, 28, 29, 30∗].

The composition of the TRAPP complex varies among species (Table 1). In S. cerevisiae, two TRAPP complexes share a hetero-heptameric core consisting of Bet3 (two copies), Bet5, Trs20, Trs23, Trs31, and Trs33 [31]. TRAPPII contains four additional proteins (Trs120, Trs130, Trs65, and Tca17) [14], and TRAPPIII has one additional Trs85 [32]. The composition of TRAPPII in A. nidulans, another fungus, is the same as that in S. cerevisiae, but its TRAPPIII contains Tca17 and Trs85 together with additional three subunits TRAPPC11, TRAPPC12 and TRAPPC13, which are absent in S. cerevisiae [17]. Compared with fungal TRAPPII, TRAPPII in metazoans (Drosophila and human) lacks Trs65 [33]. The TRAPPIII composition in metazoans is the same as that in A. nidulans [17,33]. Despite the different compositions, the essentiality for viability of subunits is also differ between species [17,33, 34, 35]. Mutations in most core subunits in all species result in lethality, although Trs33 is essential in Drosophila but nonessential in fungi [17,33,35]. Strikingly, the two TRAPPII specific subunits Trs120 and Trs130 are essential in fungi but nonessential in metazoans [17,33,35]. Conversely, the TRAPPIII specific subunit Trs85 is nonessential in fungi but essential in metazoans [17,33,35,36]. The limited conservation of the essentiality for viability of TRAPP subunits from yeast to human suggests that the regulation of TRAPP complex activities may be different among species. Indeed, the molecular mechanisms that mediate GEF specificity of TRAPP complexes, such as the specific activation of Rab11 by TRAPPII but not TRAPPIII, are critical for understanding their roles in membrane trafficking and are not fully characterized.

In the past few years, many studies have reported on TRAPP complexes. In this review, we will focus on recent structural studies of TRAPPII and their complex with the substrate Ypt32 (the Rab11 paralog in yeast), which can help us understand the mechanism of the specific activation of Rab11/Ypt32 by TRAPPII.

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