Final answer:
Ubiquitin is the marker that signals proteins for degradation by proteasomes. It tags proteins with a 'death mark' and the tagged proteins are subsequently broken down inside the proteasome after a series of steps involving ubiquitin-activating and ubiquitin-conjugating enzymes.
Step-by-step explanation:
The marker that directs proteins to the proteasomes for degradation is ubiquitin. The process begins with the attachment of ubiquitin to the target protein, signalling that the protein's lifespan is complete. The poly-ubiquitinated protein then recognizes and binds to the 19S 'CAP' structure of the proteasome. Subsequently, the target protein is digested into short peptide fragments by proteolytic enzymes inside the proteasome core. The ubiquitin-proteasomal degradation pathway involves the activation of ubiquitin by a ubiquitin-activating enzyme (E1), which, with the help of ATP hydrolysis, binds ubiquitin, followed by the ubiquitin-conjugating enzyme (E2), and finally, the protein destined for destruction binds, replacing the ubiquitin-conjugating enzyme.
Ubiquitin serves as a crucial component in the regulation of gene expression by controlling protein degradation. This system ensures that damaged or unnecessary proteins are efficiently removed from the cell, maintaining cellular health and function.