Rab GTPases and tethering in the yeast endocytic pathway

Abstract

Within eukaryotic cells, Rab GTPases control the maturation of early to late endosomes and their subsequent fusion with the vacuole. Within this ExtraView, we will focus on our recent findings regarding the activation of the Rab7 homolog Ypt7 in yeast and its interplay with the two multisubunit tethering complexes CORVET and HOPS.

Figure 1

Structural data on the two Rab GTPases Vps21 (Ypt51) and Ypt7 from S. cerevisiae. (A) secondary structure assignment and sequence alignment, (B) overlay of 3d structures. The assignments in (A) (labeled “dssp”) and the overlay in (B) were made with YASARA based on pdb entries 1ky2 and 1ek0 (www.pdb.org). The crystal structures describe residues 5–174 of Vps21 and 3–182 of Ypt7, the hypervariable domains were removed. “Switch 1” and “Switch 2” indicate the regions that change conformation most dramatically upon binding of GTP. The hypervariable domain is important in targeting and for interaction with REP and GDI (compare text). C = coil, E = β strand, H = α helix, t = turn, 3 = 3₁₀ helix. The alignment was made with TCoffee. In B Vps21 is shown in red and the bound GTP analog in yellow, Ypt7/GPPNHP in light blue/green. The N- and C-termini are colored blue, they are barely visible in the lower portion and at the back of the molecules. The hypervariable domains thus also would extend away from the viewer. The two switch regions are colored blue for Ypt7, Switch 1 is at the left border and Switch 2 at the lower left.

Figure 2

Working model of the transition from the early to the late endosome/MvB (modified from Nordmann et al.19). The activation of Vps21 takes place at the plasma membrane or endocytic vesicle via Vps9. At the early endosome, Vps21 recruits its effectors, Vps3 and Vps8, and therefore the CORV ET complex. This module is then required for the recruitment of the Mon1-Ccz1 complex, which catalyzes the nucleotide exchange of the downstream Rab GTPase Ypt7, and hence activates its signaling. It is not known so far, if the targeting of Ypt7 is supported by a GDF or possibly by a Vps21 effector. Once activated, Ypt7 binds the HOPS complex for the fusion of the MVB with the vacuole. Whether the CORV ET either matures into the HOPS, or both are recruited independently, remains to be clarified. Likewise, a specific Vps21 GAP is not known. The activation state of the Rabs is depicted by a spiny (GDP) or a smooth shape (GTP). Double-headed arrows indicate likely interactions, question marks point out gaps in our knowledge. The size of the protein icons does not reflect their respective masses.