Structural insight into the ESCRT-I/-II link and its role in MVB trafficking

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Abstract
ESCRT (endosomal sorting complex required for transport) complexes orchestrate efficient sorting of ubiquitinated transmembrane receptors to lysosomes via multivesicular bodies (MVBs). Yeast ESCRT‐I and ESCRT‐II interact directly in vitro ; however, this association is not detected in yeast cytosol. To gain understanding of the molecular mechanisms of this link, we have characterised the ESCRT‐I/‐II supercomplex and determined the crystal structure of its interface. The link is formed by the vacuolar protein sorting (Vps)28 C‐terminus (ESCRT‐I) binding with nanomolar affinity to the Vps36‐NZF‐N zinc‐finger domain (ESCRT‐II). A hydrophobic patch on the Vps28‐CT four‐helix bundle contacts the hydrophobic knuckles of Vps36‐NZF‐N. Mutation of the ESCRT‐I/‐II link results in a cargo‐sorting defect in yeast. Interestingly, the two Vps36 NZF domains, NZF‐N and NZF‐C, despite having the same core fold, use distinct surfaces to bind ESCRT‐I or ubiquitinated cargo. We also show that a new component of ESCRT‐I, Mvb12 (YGR206W), engages ESCRT‐I directly with nanomolar affinity to form a 1:1:1:1 heterotetramer. Mvb12 does not affect the affinity of ESCRT‐I for ESCRT‐II in vitro . Our data suggest a complex regulatory mechanism for the ESCRT‐I/‐II link in yeast.