# MARCH2-mediated Lys63-linked polyubiquitination promotes metastasis by modulating the catalytic activity of TGF-β type I receptor

## TL;DR

This study shows how MARCH2 modifies a receptor involved in TGF-β signaling, enhancing its activity and promoting cancer metastasis.

## Contribution

The study identifies MARCH2-mediated K63-linked ubiquitination of ALK5 as a novel mechanism regulating TGF-β signaling and metastasis.

## Key findings

- MARCH2 ubiquitinates ALK5 at lysines 342/343, enhancing its catalytic activity.
- ALK5 K342/343 mutations reduce SMAD2 phosphorylation and cell migration in A549 cells.
- ALK5 K342/343R mutant expression in mice decreases lung metastasis compared to wild-type ALK5.

## Abstract

The TGF-β signaling pathway is initiated when the type II receptor phosphorylates the type I receptor (ALK5) upon TGF-β binding. While E3 ubiquitin ligases regulate TGF-β receptor degradation, their role in modulating receptor catalytic activity via ubiquitination remains largely unexplored. Here, we demonstrate that the E3 ubiquitin ligase MARCH2 enhances ALK5 catalytic activity by conjugating K63-linked ubiquitin chains to lysines 342/343 (K342/343), primarily at endosomes following TGF-β-induced endocytosis. Mutations of ALK5 at K342/343 (K342/343R) abolish its catalytic activity for SMAD2 phosphorylation, leading to impaired TGF-β responses and reduced cell migration in A549 cells. In a mouse model, expression of the ALK5 K342/343 R mutant significantly decreases lung metastasis compared to wild-type ALK5. TCGA analysis further revealed a strong positive correlation between MARCH2 expression and TGF-β target gene expression. Collectively, these findings establish ALK5 ubiquitination at K342/343 by MARCH2 as a crucial regulatory mechanism for ALK5 catalytic activity, TGF-β signaling, and metastasis.