Receptor-Mediated Endocytosis The secretory pathway: lumenal contents from donor compartment form bud transport vesicle fusion with target compartment release INTRACELLULAR PROTEINS Gunter Blobel’s Signal Hypothesis Ribosomes in ER and cytoplasm are the same Q: How do intracellular proteins get separated from cytosol? A: Protein is synthesized on ribosome attached to ER signal sequence (“tag” on N-terminus of protein) at the beginning confers an “address ”, causing it to enter ER directly after synthesis- Blobel proposes protein-conducting channel 1. Binding of SRP (signal recognition particle) to peptide causes a pause in translation 2. SRP-bound ribosome attaches to SRP receptor in ER membrane 3. Ribosome is positioned over plug in protein translocator translation continues and translocation begins 4. Protein gets pushed into ER lumen as it’s synthesized 5. SRP and SRP receptor are discarded and replaced. Enzyme signal peptidase
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