7.88 Lecture Notes - 15 7.24/7.88J/5.48J The Protein Folding Problem Ribosomes and In Vivo Folding A. Ribosomes Ribosomes: Source of all Proteins! In prokaryotes, all ribosomes within cytoplasm; However, multiple delivery compartments: • PP chains >> cytoplasm • PP Chains >> inner membrane • PP chains >> organelles e.g. flagellum • PP chains >> through membrane to periplasm • PP chains >> through membrane and periplasm to outer membrane • PP chains >> secreted outside cell. Export pathways use protein channels - poorly defined, very active area of research; Polypeptide chains have a leader sequence whose recognition brings them to channel; this is recognized by a leader peptidase, which cleaves off leader sequence, coupled to transit. In Eukaryotes: • Cytoplasm • Exported through Endoplasmic reticuluum (Signal Recognition Particle) > Golgi> export • Imported into Mitochondria or chloroplasts • Translocated to Nucleus • Inserted into Membranes: • Many others, e.g. lysosomes Rate of Prokaryotic Cytoplasmic Protein Synthesis: Forschammer and Lindahl (1971) J. Mol. Biol. 55, 563-568. • About 20,000 ribosomes/cell in rapidly growing E. coli • Rate of synthesis = 21aa/second/ribosome in coli growing in broth at 37oC.