Vps27-Hse1 and ESCRT-I complexes cooperate to increase efficiency of sorting ubiquitinated proteins at the endosome

TLDR

Ubiquitin tagging of cell surface proteins targets them for lysosomal degradation by incorporation into multivesicular body membranes, and two yeast endosomal complexes—Vps27‑Hse1 and ESCRT‑I—have been proposed as ubiquitin‑sorting receptors. The authors mapped ubiquitin‑binding surfaces on Vps27 and Vps23 using NMR spectroscopy and mutagenesis. Mutations that abolish either Vps27 or Vps23 binding, or disrupt Vps27‑Vps23 or Vps27‑Hse1 interactions, impair ubiquitin‑mediated sorting and MVB formation, demonstrating that both complexes cooperate for efficient recognition of ubiquitinated cargo.

Abstract

Ubiquitin (Ub) attachment to cell surface proteins causes their lysosomal degradation by incorporating them into lumenal membranes of multivesicular bodies (MVBs). Two yeast endosomal protein complexes have been proposed as Ub-sorting “receptors,” the Vps27-Hse1 complex and the ESCRT-I complex. We used NMR spectroscopy and mutagenesis studies to map the Ub-binding surface for Vps27 and Vps23. Mutations in Ub that ablate only Vps27 binding or Vps23 binding blocked the ability of Ub to serve as an MVB sorting signal, supporting the idea that both the Vps27-Hse1 and ESCRT-I complexes interact with ubiquitinated cargo. Vps27 also bound Vps23 directly via two PSDP motifs present within the Vps27 COOH terminus. Loss of Vps27-Vps23 association led to less efficient sorting into the endosomal lumen. However, sorting of vacuolar proteases or the overall biogenesis of the MVB were not grossly affected. In contrast, disrupting interaction between Vps27 and Hse1 caused severe defects in carboxy peptidase Y sorting and MVB formation. These results indicate that both Ub-sorting complexes are coupled for efficient recognition of ubiquitinated cargo.