Writer｜Rachel Du
Layout Designer｜Cecilia Qin
Eukaryotic cells contain a macromolecular protease known as the proteasome, which is capable of breaking down proteins that are modified by ubiquitin, but the proteasome can also degrade proteins that lack ubiquitin modification, but the exact mechanism of this process is still unclear. Recently researchers discovered a protein known as Midnolin, which is located in the nucleus and promotes the degradation of many transcriptional regulators through the proteasome, but these transcriptional regulators are not tagged with ubiquitin. Experiments have shown that Midnolin binds tightly to the proteasome and utilizes a mechanism that includes a free beta chain domain to "trap" substrates for catabolism. Thus, the Midnolin-proteasome pathway bypasses the traditional ubiquitination system and achieves selective degradation of many nuclear proteins.
In mammals, transcriptional responses triggered by growth factors, neurons, and immune stimuli are mediated by a set of genes known as early genes (IEGs), which encode a family of transcription factors that include Fos, EGR, and NR4A. IEG proteins are activated in virtually all mammalian cells and promote the transcription of late response genes (LRGs), which are essential for the type-specific initial stimulus-response of cells. specific initial stimulus-response is critical. Thus, aberrant IEG expression is associated with cancer, immunodeficiency, and neurological disorders.IEG mRNA accumulates within a short time after initial stimulation, and once translated, its proteins are rapidly degraded to achieve a transient burst of protein expression. Although the mechanism of IEG transcriptional regulation is well understood, the mechanism by which IEG proteins are rapidly and specifically degraded has remained unsolved for many years.
Eukaryotic cells rely on the proteasome, a macromolecular protease, to efficiently degrade ubiquitin-tagged proteins. It has been proposed that Fos family proteins may undergo both ubiquitination-dependent and ubiquitin-independent mechanisms upon entry into the proteasome, but the coordination of these molecular processes remains elusive. The study hypothesized the existence of a cellular pathway capable of rapidly degrading c-Fos and other IEG proteins. By utilizing a forward genetic screen, the study aims to reveal the mechanisms that control the degradation of these proteins.
The study used a genome-wide CRISPR-Cas9 screen to explore genes that regulate the stability of IEG proteins. The results identified a protein called Midnolin in mammals that promotes proteasomal degradation in IEG proteins (e.g., c-Fos, FosB, EGR1, and NR4A1), which is not dependent on ubiquitination. In addition, Midnolin causes the degradation of a variety of other proteins, including specific transcriptional regulators in the nucleus. multiple stimuli of IEG also activate Midnolin expression, and overexpression of Midnolin induces the target to undergo ubiquitin-independent degradation .
In addition, Midnolin uses its "Catch" structural domain to engage the substrate, which is necessary and sufficient for interaction with unstructured regions within the substrate that has the potential to form a β-chain upon binding Midnolin. In addition, Midnolin binds stably to the proteasome via the C-terminal α-helix and promotes the degradation of Catch-bound targets via its N-terminal ubiquitin-like structural domain. Thus, Midnolin contains three conserved structural domains that, by acting synergistically, are able to target a large number of nuclear proteins directly to the proteasome for ubiquitination-independent degradation.
The study suggests that the Midnolin-proteasome pathway may be a general mechanism for bypassing the classical ubiquitination system in the selective degradation of nuclear proteins, especially those that are essential for transcription. Midnolin recognizes amphipathic regions within the substrate that have the simplicity to potentially form a β-strand, and thus its degradation determinants may be common structural components. Future studies need to explore how the Midnolin-proteasome pathway is regulated by multiple signals in different cell types to control transcriptional programs.
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