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Chapter 26 Protein Sorting Chapter Objectives • Understand the pathways of cotranslational processing of proteins – ER, Golgi, Plasma membrane, Lysosomes • Understand the pathways of posttranslational processing of proteins – Mitochondria, Peroxisomes, Nucleus Overview • Cytoplasmic proteins • Mitochondria, peroxisome, or nuclear proteins • Extracellular, lysosomal, ER proteins Cell Reminder ER Golgi How do proteins know where to go? • Signal Peptide – Sends proteins to ER – No signal, to the cytosol • Blobel 1999 Nobel Prize Some polar Mature protein N Hydrophobic (10-15 aa) Cleavage site To the ER! GDP GTP GDP Oligosaccharides – N-linked Goodbye ER, Hello Golgi • Must physically cross a space. • COP-coated vesicles – Coat protein complex • COP-II sends vesicles to Golgi Finding a Target • ARF GTPase activity activated at target – Coat is removed • Target – Plasma membrane • Nerve cells – Other organelles Oops!! Goodbye Golgi, Hello ER • COP-I • KDEL sequence in protein Review To the Lysosome • Vesicles bud from Golgi to a variety of places – Lysosomes • Termernal glycosylation is mannose 6P • Destructive Clathrin-coat • Another way to make vesicles – Dynamin pinches vesicle • GTP dependent – Clathrin-coated vesicles become endosomes • Endosomes can become or fuse with lysosomes. Integral Membrane Proteins To the Mitochondria To the Peroxisome • Peroxisomes are thought to be artifact organelles – Oxidation reactions • Without producing energy – Break down lipids – No synthesis of proteins – PTS1 and PTS2 are signals to import proteins • PTS1 has a c-terminal SKL tripeptide To the Nucleus • Transport proteins – Proteins and RNA are leaving the nucleus – Proteins are trying to get into the nucleus – Some proteins are really big • Nuclear Pore Complex (nucleoporin) – Very big allows diffusion of up to 40,000 Da proteins – Larger proteins must be accepted by complex Nuclear Pore Complex Big Protein Transport • NLS (nuclear localization signal) – Very positive charge • PKKKRKV • KRxxxxxxxxxxPAAIKKAGQAKKKK • NES (nuclear export signal) • Some proteins go both ways – Heteroribonucleoprotein complex (hnRNP) • Shuttles mRNA out of nucleus • Returns to pick up more • 38 amino acid signal sends it both directions Entropy and Transport to Nucleus • Not energy mechanism innate to nucleoporin • Uses small GTP/GDP binding protein Ran Ran as an Energy Mechanism Controlling Export/Import
