The TRAPPIII complex activates the GTPase Ypt1 (Rab1) in the secretory pathway

Abstract

The TRAPP complexes are nucleotide exchange factors that activate Rab GTPases, and four different versions of TRAPP have been reported. Thomas et al. show that only two versions of TRAPP are detectable in normal cells and demonstrate that the TRAPPIII complex regulates Golgi trafficking in addition to its established role in autophagy.

A Steric Gating Mechanism Dictates the Substrate Specificity of a Rab-GEF

“…To test this, we performed time-lapse imaging experiments. Consistent with previous studies, mNG-Ypt1 levels peaked approximately 20 seconds upstream of Sec7, correlating with TRAPPIII-mediated nucleotide exchange (Figure 5C and D; McDonold and Fromme, 2014; Thomas et al, 2018). mNG-Ypt1 HVD32 also accumulated upstream of Sec7, but recruitment of the chimera peaked significantly later.…”

“…In contrast to TRAPPII, TRAPPIII can associate directly with TGN membranes in the absence of other recruiters (Figure S1B), and we confirmed that TRAPPIII activated Ypt1, but not Ypt32, in the presence of TGN liposomes (Figure 1E; Thomas et al, 2018). Taken together these data indicate that, under physiological conditions, the two TRAPP complexes display distinct Rab substrate specificities.…”

“…The TRAPPIII complex is distinguished by the addition of one distinct subunit whereas TRAPPII contains four additional subunits (Figure 1A). Multiple lines of evidence indicate that TRAPPIII is a specific Ypt1 GEF (Lynch-Day et al, 2010; Tan et al, 2013; Thomas et al, 2018). There is less of a consensus regarding the substrate of TRAPPII.…”

“…In agreement with previous studies, we found that wild-type mNG-Ypt32 accumulated at the TGN after Sec7, correlating with activation by TRAPPII (Figure 4C and 5D; Kim et al, 2016; McDonold and Fromme, 2014; Thomas and Fromme, 2016). In contrast, mNG-Ypt32 HVD1 arrived at the Golgi before Sec7, coinciding with peak TRAPPIII recruitment (Thomas et al, 2018), suggesting that TRAPPIII weakly activates Ypt32 HVD1 in cells. Taken together, these results indicate that the Ypt31/32 HVD is required for activation by TRAPPII in vivo .…”

“…In contrast, the majority of GEFs display substrate specificity, allowing for precise Rab targeting. Most Rabs become mislocalized in the absence of their GEFs, indicating that GEF activity is required for proper localization (Cabrera and Ungermann, 2013; Gerondopoulos et al, 2012; Morozova et al, 2006; Thomas and Fromme, 2016; Thomas et al, 2018). Moreover, several studies have shown that anchoring GEFs to mitochondria causes a corresponding relocalization of the Rab substrate, suggesting that GEF activity is sufficient for Rab targeting (Blümer et al, 2013; Gerondopoulos et al, 2012).…”