Protein-sorting pathways in eukaryotes

All nuclear-encoded mRNAs are translated on cytosolic ribosomes 1. Synthesis of proteins lacking ER signal sequence is completed on free ribosomes. 2. Proteins with organelle-specific targeting seq are first released into the cytosol, and then imported into organelles. Ribosomes synthesizing nascent proteins in the secretary pathway are directed to the rough ER by ER signal sequence. After translation is completed on ER these proteins can move through TV to Golgi complex. EM of ribosomes attached to the rough ER in a pancreatic acinar cell. It synthesizes, stores, and secretes digestive enzymes. These cells synthesize, store, and secrete digestive enzymes. Most proteins in these cells are synthesized on membrane-attached ribosomes. Free ribosomes synthesize cytosolic or non- secretory proteins. Protein synthesis on the ER Membrane-bound and free cytosolic ribosomes are identical. Membrane-bound ribosomes are recruited to the ER during protein synthesis of a polypeptide containing an ER signal sequence. Labeling experiments demonstrate that secretory proteins are localized to ER shortly after synthesis. Pancreatic acinar cells (because they make large quantities of secreted proteins) are incubated briefly with radiolabeled amino acids so that only newly synthesized proteins become labeled. The goal of this experiment was to elucidate the sequence of events that occurs immediately after synthesis of a secretory protein. Cells are homogenized, fracturing the PM and shearing the ER into small vesicles called microsomes (liposome ribosome). Because they have bound ribosomes, microsomes have higher buoyant density than other membranous organelles and can be separated from them. The purified microsomes are treated with a protease in the presence or absence of a detergent. The labeled secretory proteins associated with the microsomes are digested only if the permeability barrier of the microsomal membrane is first destroyed with a detergent. Conclusion: The newly synthesized proteins are inside the microsomes, which is equivalent to the lumen of RER. Translation and translocation occur simultaneously. #NikolaysGeneticsLessons #protein #polypeptide #aminoAcid #peptide #disulfideBridges #covalentBounding #ionicBounding #hydrophobicInteraction #proteinStructure #proteinSequence #proteinFolding #aminoAcids #proteins #Polypeptides #ImidazolRing #Histidine #polypeptideChain #oligopeptide #hydrolysesSynthesis #condensationReaction #condensationSynthesis #peptides #globularProteins #fibrousProteins #Genetics