Compare the ubiquitin-proteasome system and autophagy for protein degradation.

The ubiquitin-proteasome system (UPS) degrades short-lived, misfolded, or regulatory proteins. Proteins are tagged with polyubiquitin chains through E1-E2-E3 enzyme cascade, then recognized and degraded by the 26S proteasome into peptides. UPS provides selective, rapid degradation for cell cycle regulators, transcription factors, and damaged proteins. Autophagy degrades long-lived proteins and organelles through formation of autophagosomes that fuse with lysosomes. Types include macroautophagy (bulk), microautophagy, and chaperone-mediated autophagy. Autophagy provides building blocks during starvation and removes damaged organelles. Both systems maintain proteostasis; dysfunction contributes to aging and disease.