Ubiquitiin

Ubiquitin is a small, highly conserved protein found in almost all eukaryotic organisms. Its primary role is as a post-translational modifier, meaning it is attached to other proteins after they have been synthesized. This attachment, known as ubiquitination, can occur as a single molecule (monoubiquitination) or as a chain of ubiquitin molecules (polyubiquitination). The nature of the ubiquitin chain dictates the fate of the target protein.

Polyubiquitination, particularly chains linked through the K48 residue of ubiquitin, is a well-established signal for protein degradation. These tagged proteins are typically recognized and degraded by the proteasome, a cellular machine responsible for protein turnover. This process is crucial for removing damaged or misfolded proteins, as well as for regulating the levels of key cellular proteins involved in processes like cell cycle control and signal transduction.

However, ubiquitination has diverse functions beyond degradation. Monoubiquitination and polyubiquitination chains linked through other lysine residues (e.g., K63) can act as signaling scaffolds, recruit other proteins to specific cellular locations, or alter the activity of the target protein. These non-degradative roles are involved in DNA repair, protein trafficking, immune responses, and signal transduction pathways. The ubiquitin system is therefore a highly dynamic and versatile regulatory mechanism essential for cellular function and homeostasis.