# Structure and mechanism of the HECT ligase HECTD3

## TL;DR

This paper reveals the structure and function of HECTD3, a cancer-related enzyme that modifies proteins with ubiquitin chains, using cryo-EM and biochemical analysis.

## Contribution

The study provides full-length cryo-EM structures of HECTD3 and identifies its unique domain organization and ubiquitin linkage preferences.

## Key findings

- HECTD3 has a distinct N-terminal domain fold and unique catalytic features.
- The enzyme preferentially synthesizes specific ubiquitin chain types.
- Structural insights reveal how HECTD3 recognizes substrates and regulates ubiquitination.

## Abstract

HECT E3 ligases regulate many cellular processes, yet how they recognise their substrates and synthesise specific types of poly-ubiquitin chains is still incompletely understood. HECTD3, a member of the “other HECT” family, is implicated in the regulation of inflammation, apoptosis, and infection and highly expressed in several cancers. These functions are largely attributed to its ligase activity and modification of diverse substrates with different types of ubiquitin chains. We present a detailed analysis of the ligase activity of HECTD3, including its ubiquitin linkage preferences, oligomeric state and substrate ubiquitination. Using cryo-EM, we provide the full-length structures of HECTD3 in both apo and ubiquitin-loaded forms, revealing key insights into its domain organisation, including discovery of a distinct fold of the N-terminal region, and mechanistic features. Some of these are shared with other HECT ligases, while others are unique to HECTD3 and contribute to differences in its catalytic mechanisms and functional diversity.

HECT-type ubiquitin ligases control many cellular processes but remain poorly characterised in their full-length form. Here, the authors present a biochemical analysis and cryo-EM structures of the cancer-associated ligase HECTD3 revealing an interesting domain architecture and mechanistic insights.