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Lecture 9

Lecture 9.docx

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Department
Biology
Course Code
BIO230H1
Professor
Darrel Desveaux

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Description
Lecture 9: Protein Sorting BIO230 Oct 8, 2013 protein sorting: movement of proteins - within cell from different compartments, out of cell, or into cell two types of protein sorting: 1. post-translational process: (protein fully synthesized in cytosol before sorting) and then protein sorted to proper organelle. protein can be maintained unfolded plastids = chloroplasts 2. co-translational process: proteins can already be folded. proteins w Endoplasmic Reticulum (ER) signal sequence associated with ER during protein synthesis as protein synthesized, injected into ER (endoplasmic reticulum) PROTEIN SORTING: MECHANISMS (3 types) 1. gated transport movement of proteins btw cytosol and nucleus nuclear pore complex is the gate that selectively choses which proteins are allowed inside or out. pores in nuclear membrane - does not allow passive transport in and out of nucleus. form channels. allows for selective transport of macromolecules. free diffusion of micro molecules (if smaller than 5 000 Daltons). move through nuclear pore complexes. nuclear pore complex only selective for larger molecules bigger than 5 000 Daltons. proteins tend to be macromolecules. nuclear import - movement of proteins through nuclear pore complex from cytosol to nucleus nuclear export - movement of proteins from nucleus to cytosol nuclear envelope - lipid bilayer surrounding nucleus. not all proteins allowed to move through NPC - only ones that have specific receptors. NUCLEAR IMPORT - proteins coming in are called cargo proteins. have specific signal on them (nuclear localization signals). amino acid sequence is NLS. nuclear import receptor moves protein from cytosol to nucleus. it binds to amino acid sequence NLS. not specific sequence, but they are rich in lysine and arginine. once receptor binds to NLS, it binds to nucleoporins (are proteins) in nuclear pore complex. can have different import receptors that regulate movement of proteins. and receptors vary because NLS sequences vary. once cargo bound to receptor and receptor bound to nucleoporins, then transported from nucleus to cytosol. NUCLEAR EXPORT cargo proteins move out of nucleus have nuclear export signal (usually hydrophobic amino acids). i.e.: ribosomes assembled in nucleus have nuclear export signals. also, RNA molecules. nuclear export receptor - binds to Nuclear Export Signal binds to nucleoporins in Nuclear Pore Complex IMPORT AND EXPORT maintained by plasma proteins called Ran GTPases. play role in import and export. RAN GTPase - hydrolyzes GTP to GDP. RAN GTPase cycles btw GTP-bound and GDP-bound states. RAN-GAP regulates GTPase. it stimulates GTP hydrolysis by RAN- GAP. Ran-GEF promotes exchange of GTP for GDP by Ran-GAP. RAN-GAP localized in cytosol. and RAN-GEF in nucleus. high [] RAN-GAP in cytosol therefore high RAN-GTP in nucleus high [] RAN-GEF in nucleus therefore low [] Ran-GTP in cytosol critical for direction of transport Ran-GTP carries with it nuclear export receptors to cytosol and RAN-GDP carries with it NTF2 to nucleus NUCLEAR IMPORT OF CARGO movement from cytosol to nucleus nuclear import receptor binds to cargo in cytosol receptor and cargo then move through nuclear pore complex into nucleus Ran-GTP binding causes cargo release now, you need to recycle import receptor. empty import receptor and Ran-GTP move to cytosol. Ran-Binding protein and Ran-GAP promote: -GTP hydrolysis (GTP->GDP) -and release of empty import receptor NUCLEAR EXPORT OF CARGO nuclear export receptor - binds to Ran-GTP and cargo in nucleus receptor has NLS that allows it to bind Ran-GTP predominantly found in nucleus NTF2 moves Ran-GDP to nucleus REGULATED TRANSPORT IN AND OUT OF NUCLEUS mo
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