# UBE4B Mediates Mitophagy via NIPSNAP1 Ubiquitination and NDP52 Recruitment

## TL;DR

This study reveals a new pathway for mitophagy involving UBE4B and NIPSNAP1, which helps maintain cellular health when the usual Parkin pathway is absent.

## Contribution

The discovery of UBE4B as an E3 ubiquitin ligase for NIPSNAP1 and its role in a Parkin-independent mitophagy pathway.

## Key findings

- UBE4B catalyzes NIPSNAP1 ubiquitination in HEK293T and HeLa cells.
- UBE4B enhances NIPSNAP1 interaction with NDP52 and p62/SQSTM1 under mitochondrial depolarization.
- UBE4B-mediated ubiquitination supports mitophagy in Parkin-null HeLa cells.

## Abstract

Mitophagy, as a critical form of selective autophagy, plays a central role in maintaining cellular homeostasis. While the canonical PTEN-Induced Kinase 1 (PINK1)–Parkin pathway is well established, mitophagy can still be effectively induced in Parkin-deficient cells such as HeLa, indicating the existence of Parkin-independent alternative pathways. The mitochondrial matrix proteins 4-Nitrophenylphosphatase domain and non-neuronal SNAP25-like protein homolog 1 (NIPSNAP1) acts as a key effector in such pathways, yet its regulatory mechanisms remain incompletely understood. Here, we identify Ubiquitination Factor E4B (UBE4B) as an E3 ubiquitin ligase for NIPSNAP1 and demonstrate that it catalyzes NIPSNAP1 ubiquitination in both Human Embryonic Kidney 293 cells (HEK293T) and HeLa cells. Under mitochondrial depolarization, UBE4B not only promotes NIPSNAP1 ubiquitination and subsequent lysosome-dependent degradation, but also significantly enhances its interaction with the autophagy adaptors Nuclear Dot Protein 52 kDa (NDP52) and Sequestosome 1 (p62/SQSTM1). Notably, while Parkin does not ubiquitinate NIPSNAP1, UBE4B-mediated ubiquitination facilitates mitophagy in Parkin-null HeLa cells by strengthening the binding between NIPSNAP1 and NDP52. Collectively, this study unveils a novel mitophagy pathway regulated by the UBE4B-NIPSNAP1 axis, offering new insights into mitochondrial quality control.