What is the function of ubiquitin in cellular processes?

Ubiquitin plays a crucial role in cellular processes by tagging proteins for degradation, which is essential for maintaining protein homeostasis within the cell. This small protein is involved in the ubiquitin-proteasome pathway, where it attaches itself to target proteins, marking them for transport to the proteasome. The proteasome then degrades these ubiquitin-tagged proteins into smaller peptides and amino acids, which can be recycled by the cell.

This tagging process not only regulates protein turnover but also plays a role in controlling various cellular functions, such as the cell cycle, signal transduction, and responses to stress. By ensuring that damaged or misfolded proteins are removed, ubiquitin helps prevent cellular dysfunction and contributes to various signaling pathways.

The other options pertain to different cellular functions. DNA repair is governed by a different set of proteins and mechanisms, lipid synthesis involves enzymes specific to lipid metabolism, and rRNA binding relates to ribosomal assembly and function, separate from the ubiquitin-mediated process. Thus, the primary and well-established role of ubiquitin is in marking proteins for degradation, making it central to cellular regulation and maintenance.