HECTD3 is a small HECT E3 ligase with unique features

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 first full-length structures of HECTD3 in both apo and ubiquitin-loaded forms, revealing key insights into its domain organisation, including discovery of a novel fold of the N-terminal region, and mechanistic features. Some of these are shared with other HECT ligases, while others are unique to HECTD3, offering new perspectives on its catalytic mechanisms and functional diversity.