Ubiquitin modifications are a dynamic post-translational modification involved in nearly all aspects of eukaryotic biology. Once attached to a substrate, ubiquitin can undergo further modifications, creating a 'ubiquitin code' with distinct cellular outcomes. These modifications include ubiquitination on seven lysine residues or the N-terminus, leading to polyubiquitin chains with complex topologies. Additionally, ubiquitin can be modified by ubiquitin-like molecules (e.g., SUMO, NEDD8) or by acetylation or phosphorylation on specific residues. These modifications can dramatically alter signaling outcomes. The ubiquitin system is highly complex, with over 1,000 proteins regulating ubiquitination in human cells. Ubiquitination occurs through a three-step enzymatic cascade involving E1, E2, and E3 enzymes. Ubiquitinated proteins are recognized by receptors with ubiquitin-binding domains, and deubiquitinases (DUBs) remove these modifications. Proteomics studies have identified tens-of-thousands of ubiquitination sites on thousands of proteins, indicating that most proteins experience ubiquitination during their cellular lifetime. Monoubiquitination is abundant and has many roles in cells. Further modifications of monoubiquitin can generate a wide range of signals, leading to the concept of the 'ubiquitin code'. This code includes modifications such as phosphorylation and acetylation, which can provide additional regulation in the ubiquitin system. However, the enzymes and receptors that recognize or remove these modifications are largely unknown. Recent studies have highlighted the role of ubiquitin acetylation and phosphorylation, particularly Ser65-phosphorylation, in mitophagy and Parkin activation. Mitophagy is the process by which cells selectively destroy damaged mitochondria. PINK1, a mitochondrial kinase, plays a key role in this process. Upon mitochondrial damage, PINK1 accumulates on damaged mitochondria and phosphorylates Parkin on Ser65, leading to its activation and subsequent ubiquitination of damaged mitochondria. This process is essential for mitochondrial quality control and the prevention of Parkinson's disease. The ubiquitin system is highly dynamic, with various modifications influencing chain architecture and function. The 'ubiquitination threshold' model suggests that the amount of polyubiquitin rather than the type is important for proteasomal degradation. This model is supported by recent findings showing that multiple short chains or branched ubiquitin are more efficient degradation signals than a single Lys48-linked tetraubiquitin. In addition to ubiquitin chains, other modifications such as SUMOylation and NEDDylation can also modify ubiquitin, adding complexity to the system. These modifications can influence the function and stability of ubiquitin and its interactions with other proteins. The study of these modifications is crucial for understanding the role of ubiquitin in various cellular processes, including protein degradation, signaling, and disease.Ubiquitin modifications are a dynamic post-translational modification involved in nearly all aspects of eukaryotic biology. Once attached to a substrate, ubiquitin can undergo further modifications, creating a 'ubiquitin code' with distinct cellular outcomes. These modifications include ubiquitination on seven lysine residues or the N-terminus, leading to polyubiquitin chains with complex topologies. Additionally, ubiquitin can be modified by ubiquitin-like molecules (e.g., SUMO, NEDD8) or by acetylation or phosphorylation on specific residues. These modifications can dramatically alter signaling outcomes. The ubiquitin system is highly complex, with over 1,000 proteins regulating ubiquitination in human cells. Ubiquitination occurs through a three-step enzymatic cascade involving E1, E2, and E3 enzymes. Ubiquitinated proteins are recognized by receptors with ubiquitin-binding domains, and deubiquitinases (DUBs) remove these modifications. Proteomics studies have identified tens-of-thousands of ubiquitination sites on thousands of proteins, indicating that most proteins experience ubiquitination during their cellular lifetime. Monoubiquitination is abundant and has many roles in cells. Further modifications of monoubiquitin can generate a wide range of signals, leading to the concept of the 'ubiquitin code'. This code includes modifications such as phosphorylation and acetylation, which can provide additional regulation in the ubiquitin system. However, the enzymes and receptors that recognize or remove these modifications are largely unknown. Recent studies have highlighted the role of ubiquitin acetylation and phosphorylation, particularly Ser65-phosphorylation, in mitophagy and Parkin activation. Mitophagy is the process by which cells selectively destroy damaged mitochondria. PINK1, a mitochondrial kinase, plays a key role in this process. Upon mitochondrial damage, PINK1 accumulates on damaged mitochondria and phosphorylates Parkin on Ser65, leading to its activation and subsequent ubiquitination of damaged mitochondria. This process is essential for mitochondrial quality control and the prevention of Parkinson's disease. The ubiquitin system is highly dynamic, with various modifications influencing chain architecture and function. The 'ubiquitination threshold' model suggests that the amount of polyubiquitin rather than the type is important for proteasomal degradation. This model is supported by recent findings showing that multiple short chains or branched ubiquitin are more efficient degradation signals than a single Lys48-linked tetraubiquitin. In addition to ubiquitin chains, other modifications such as SUMOylation and NEDDylation can also modify ubiquitin, adding complexity to the system. These modifications can influence the function and stability of ubiquitin and its interactions with other proteins. The study of these modifications is crucial for understanding the role of ubiquitin in various cellular processes, including protein degradation, signaling, and disease.