Final answer:
Ubiquitination can occur on various lysine residues, and the polyubiquitin chain formation is the signal for protein degradation by the proteasome, a process that regulates protein lifespan and gene expression.
Step-by-step explanation:
The lysine that is ubiquitinated for lysosomal degradation is not generally specified because ubiquitination can occur at various lysine residues. The form of ubiquitination involved in targeting proteins for degradation is a process in which multiple ubiquitin molecules are attached to a protein, leading to the formation of a polyubiquitin chain. This chain serves as a signal for the protein to be recognized and degraded by the proteasome. The proteasome is a large protein complex that degrades unwanted or damaged proteins after they have been tagged with ubiquitin, thus controlling protein lifespan and, in turn, gene expression.
Proteins are tagged for degradation by a cascade of enzymes: the ubiquitin-activating enzyme (E1), ubiquitin-conjugating enzyme (E2), and the ubiquitin ligases (E3). The E3 ligase confers specificity to the process, determining which protein will be ubiquitinated. Once polyubiquitinated, the proteins are delivered to the proteasome, unfolded, and then digested into peptide fragments. These fragments can then be further broken down into amino acids in the cytoplasm, contributing to the recycling of these basic biological building blocks.