A recent study published in Protein & Cell by researchers from the Southern University of Science and Technology (SUSTech) provides critical structural and functional insights into the UBR4-KCMF1-Calmodulin (UKC) complex, revealing its pivotal role in protein quality control, cancer cell survival, and stress adaptation. The work, led by Professors Renhong Yan and Baotong Zhang, elucidates how this complex assembles and functions as an E4 ubiquitin ligase, extending ubiquitin chains on pre-tagged substrates to regulate protein degradation.

The UKC complex is a stress-responsive machinery that suppresses the integrated stress response triggered by mitochondrial import stress. Using cryo-electron microscopy, the team determined the structure of the human UKC complex at 3.89 Å resolution, uncovering a toroidal architecture where KCMF1 sits within the UBR4 scaffold ring. Calmodulin (CaM), a calcium-sensing protein, binds asymmetrically to the complex, with its C-terminal lobe adopting an open conformation to engage UBR4, while the N-terminal lobe remains semi-closed. This calcium-dependent interaction is crucial for the complex's ubiquitination activity, as EGTA chelation significantly reduced its enzymatic function.

Functional assays demonstrated that the UKC complex preferentially employs UBE2A as its E2 enzyme and requires substrates carrying a pre-attached ubiquitin moiety for efficient modification. This E4-like activity was confirmed using mitochondrial targeting sequences (MTS) from ACOT9, COX8A, COQ9, and NIPSNAP1, which were ubiquitinated only in their ubiquitin-fused forms. KCMF1 emerged as a central player in both complex assembly and catalytic activation. Its ZZ-DZB region recognizes pre-ubiquitinated substrates, while its C-terminal helix maintains structural integrity. Mutations in the ZZ-DZB region impaired UBR4 binding and ubiquitination activity, highlighting KCMF1 as a critical node linking ubiquitin signaling to cellular fitness.

The study also established the biological relevance of the UKC complex in prostate cancer. Depletion of UBR4 or KCMF1 significantly inhibited tumor growth in mouse xenograft models and sensitized cancer cells to chemotherapeutic agents like docetaxel and cisplatin. Overexpression of UBR4 induced degradation of key substrates such as ACLY, DELE1, and HRI, a process dependent on an intact UBR4-KCMF1 interface. Notably, disease-associated UBR4 variants near the CaM-binding region, including R4111H linked to neurological disorders, suggest that defective calcium-dependent regulation may contribute to both neuronal dysfunction and tumor biology.

This work provides a molecular framework for understanding how the UKC complex coordinates protein degradation and stress signaling. By defining the distinct roles of UBR4 as a scaffolded E4 ligase and KCMF1 as a substrate-processing factor, the study opens avenues for targeted therapies. Disrupting the KCMF1-substrate interface may offer a more selective strategy for treating prostate cancer and neurodegenerative diseases, offering hope for more effective interventions against these challenging conditions.

DOI:10.1093/procel/pwag036