Heavy Metal Meets Protein Homeostasis: Emerging Roles of F‐Box Proteins?

Protein degradation has become an area of focus for modern research due to the technological advancement of targeted protein degradation as a highly specific and selective therapeutic approach [1]. However, as can be said of most scientific discoveries, the cell itself has long utilized and fine-tuned this regulatory process to create intricate layers of regulation surrounding protein degradation to attune the proteome to intracellular and extracellular cues. Thus far, the answer to this question has indicated that post-translational modifications (PTMs) on target proteins are responsible for tuning protein degradation to the cell's needs [2]. As our understanding of protein degradation delves further into the mechanistic and biological implications of the relationship between the cellular environment, PTMs, and protein degradation, further questions are uncovered regarding why PTMs have evolved to play these central roles in protein degradation. This hypothesis by Hwang et al., [3] takes a refreshing perspective on this next step: Can the ubiquitination machinery itself be directly subject to environmental cues to add an additional regulatory layer to protein degradation? Protein degradation largely occurs through the ubiquitin-proteasome system (UPS). The UPS utilizes a three-step enzymatic cascade to processively attach ubiquitin (Ub) onto surface lysine residues of proteins targeted for degradation. The ubiquitination process is highly coordinated, thereby allowing for regulation at almost every step before, during, and after the ubiquitination cascade. The most common regulatory step occurs at ubiquitin attachment, wherein E3 ligase complexes function to coordinate transfer of the ubiquitin from an E2 ubiquitin-conjugating enzyme onto the target protein. These complexes are often made up of multiple adaptor and scaffold proteins, again indicating the level of regulation required for ubiquitination. Cullin-RING ligases (CRLs), the majority of known E3 ligases, consist of a Cullin scaffold protein and a RING adaptor protein which functions to bind the E2-Ub complex. Families of CRLs are characterized based on the other proteins that make up the complex. Skp1-cullin1-Fbox (SCF) are a subset of CRLs that utilize adaptor protein Skp1 to bind an F-box protein (FBP) bound to the target protein [4]. Intuitively, the recruitment of the target protein to the SCF complex via FBPs is crucial for protein ubiquitination via SCF complexes. This article posits the hypothesis that FBPs function as environmental sensors to control stress response mechanisms, particularly in response to heavy metals [3]. FBPs are well-evidenced to respond to various PTMs (including but not limited to phosphorylation, methylation, and acetylation), which are arguably all ways in which the cell can transmit environmental signals to protein degradation. The authors emphasis of hydroxylation [5] within this context is, therefore, an expansion on well-known regulatory processes dictating how PTMs allow FBP-target protein interactions to remain sensitive to environmental perturbations. The author's expansion into heavy metal sensing, however, is an exciting new hypothesis that is so novel that it is potentially under-evidenced. Canonically, the FBP is made up of two domains: (1) an F-box domain highly conserved for the interaction to adaptor protein Skp1 and (2) a protein interaction domain that binds the protein targeted for degradation. The authors hypothesize that the F-box domain may also sense heavy metals with functional implications to protein degradation, in addition to being an anchor to Skp1 and the rest of the SCF complex. The foundation for this hypothesis is found in the examples of yeast FBPs outlined within the article [6, 7], leading to the suggestion that human FBPs may also contain these functions. However, initial phenotypic verification of this hypothesis is not currently available. Correlative studies demonstrating statistically significant links between SCF complex activity and/or stability in the presence of various metals via cell-wide techniques (i.e., proteomics) could address this issue. In summary, the authors present an intriguing next step for the field of protein homeostasis: the integration of findings from other species to delve into domains and mechanisms long believed to contain just one simple function. This hypothesis builds on a strong foundation of FBPs as conditionally sensitive via PTM-mediated or -hindered protein-protein interactions with proteins targeted for degradation and looks ahead to new possibilities for the regulation of protein degradation. This article comments on the hypothesis paper by Jiwon Hwang, Linda Lauinger, and Peter Kaiser https://doi.org/10.1002/bies.202400249. The authors declare no conflicts of interest.