RNF126 writes a non-canonical ubiquitin code on midnolin to tune protein stability

Abstract

Midnolin (MIDN) is a newly recognized master regulator that drives ubiquitin-independent proteasomal degradation, yet the mechanisms governing its own turnover remain enigmatic. Here, we demonstrate that MIDN is ubiquitinated and identify RNF126 as the cognate E3 ligase. RNF126 physically associates with MIDN and catalyzes its ubiquitination, and mass spectrometry mapping reveals that this process occurs primarily at non-canonical cysteine, serine, and threonine residues (C230, C236, S237, T239, and S241) rather than at lysine residues. This non-classical ubiquitination targets MIDN for 26S-proteasomal degradation. In vivo dissection of the RNF126-MIDN axis shows that it governs EGR1 abundance and, consequently, the tumor-suppressor proteins PTEN and p53, thereby restraining the progression of testicular germ-cell tumors (TGCTs). Our findings reveal an unappreciated layer of MIDN regulation and identify the RNF126-MIDN ubiquitination cascade as a potential therapeutic vulnerability in TGCTs and related malignancies.

1. Introduction

Protein homeostasis underpins every cellular decision—whether division, differentiation, stress adaptation, or death—by maintaining the proteome in precise balance across its abundance, quality, spatial distribution, and temporal dynamics [1–5]. Its dysregulation underpins a spectrum of human diseases, including cancers and diverse neurological disorders [6–12]. The ubiquitin-proteasome system (UPS) is the principal engine of proteostasis in eukaryotes. By covalently attaching the 76-amino-acid protein ubiquitin to internal lysine residues of substrate proteins, E3 ubiquitin ligases generate a combinatorial code (mono-, multi or polyubiquitin chains of varying linkage topology) that is decoded by proteasomal receptors, de-ubiquitinating enzymes and other effectors [2,13]. Canonical K48-linked polyubiquitin chains are the best-characterized signal for 26S proteasomal degradation, but noncanonical linkages (K11, K63, M1, etc.) and non-lysine acceptor sites are increasingly recognized as vital layers of regulatory complexity [2,14,15].

Midnolin (MIDN) has recently emerged as a fascinating nexus between transcriptional regulation and protein stability. Initially annotated as a nucleolar protein, MIDN unexpectedly drives the 26S proteasomal degradation of a select cohort of transcription factors (e.g., EGR1, FosB, and NR4A1) in a ubiquitination-independent manner [16,17]. Through this ubiquitination-independent proteasomal activity, MIDN governs distinct developmental and pathological programs—neuronal lineage progression, T-cell immune responses, and oncogenic signaling by selectively targeting discrete substrate transcription factors in each context [18–20]. Given the inherently short half-life of MIDN, its intracellular abundance must be stringently controlled; however, the molecular machinery underpinning this regulation remains to be elucidated.