organization of cell
DESCRIPTION
Organization Of Cell. Contains membrane-enclosed organelles Nucleus Cytoplasm. Cytoplasmic organelles Cytosol. Transporting Proteins To Organelles. Synthesis begins in cytosol Several mechanisms for transporting to organelles. Sorting Signals. - PowerPoint PPT PresentationTRANSCRIPT
Organization Of Cell
• Contains membrane-enclosed organelles
• Nucleus• Cytoplasm
– Cytoplasmic organelles– Cytosol
Transporting Proteins To Organelles
• Synthesis begins in cytosol• Several mechanisms for
transporting to organelles
Sorting Signals
• Segment(s) of amino acids direct protein to an organelle• Recognized by sorting receptors• Proteins with no sorting signal remain in cytosol
Examples Of Signal Sequences
• Sequence variability; physical properties often important
Studying Signal Sequences
• Functional signal sequences determined by experimental manipulation of proteins to alter their localization
Nuclear Pore Complex
• Multi-protein complex composed of nucleoporins• Diffusion of small molecules• Selective gate for proteins
Nuclear Import & Export
• Nuclear import receptor binds NLS of protein to be imported• Cargo-bound import receptor binds nucleoporins• Nuclear export is similar: export receptor binds to NES
Functions Of Peroxisomes
• Enzymes produce and consume H202 to oxidize organic substrates
RH2 + O2 → R + H202 (various enzymes)
H202 + R'H2 → R' + 2H20 (catalase)2 H202 → 2H20 + O2 (catalase)
• Synthesis of plasmalogens
Import Into Peroxisomes
• Signal sequence often at C-terminus• Some proteins with sequence near N-terminus• Peroxins (receptors, docking proteins) participate in transport• Inherited defects in peroxin genes such as Zellweger syndrome
Transport Into Mitochondria
• Have own genome for some proteins; maternally inherited
• Nuclear genome encodes most proteins; synthesized in cytosol and imported
Endoplasmic Reticulum
• Site of synthesis for all proteins destined for secretion, the plasma membrane, lysosomes, endosomes, the Golgi, or the ER itself
Docking Protein Onto ER Membrane
• Signal sequence contains hydrophobic amino acids• SRP binds to signal sequence as it emerges from
ribosome• Co-translational transport onto ER membrane• Start transfer through translocator as translation continues
Soluble Protein Into ER Lumen
• Signal sequence at N-terminus• Co-translational transport and translocation through membrane• Cleavage of signal sequence
ER Transmembrane Protein With N-Terminal Signal
– Acts to stop transfer– Remains as membrane-spanning segment
• N-terminal sequence for transport and start transfer• Additional internal hydrophobic segment
ER Transmembrane Protein With Internal Signal
• Internal sequence for transport and start-transfer• Remains as membrane-spanning segment• Two orientations of signal sequence
ER Multi-pass Transmembrane Protein
• Multiple internal start and stop tranfer sequences
N-Linked Glycosylation
• Glycoproteins made in ER• Oligosaccharide precursor added
to asparagine residues in ER • Processing in Golgi removes
some sugar residues
• Transfer of preformed oligosaccharide precursor
• Catalyzed by oligosaccharyl transferase
• Oligosaccharide to be transferred attached to dolichol
Glycosylation In ER
• Stepwise addition of sugar resides
• Nucleotide-sugar intermediates donate sugars
• Monosaccharide-linked dolichol molecules transfer sugars
Synthesis Of Dolichol-linked Oligosaccharide
O-Linked Glycosylation
• Oligosaccharide linked to hydroxyl groups of serine, threonine, or hydroxylysine residues
• Occurs in Golgi
Protein Folding In ER
• Chaperones aid in folding• Improperly folded proteins enter cytosol through
translocator; deglycosylated, ubquitylated, and degraded
• Some proteins destined for plasma membrane• Hydrophobic C-terminal sequence• C-terminus cut and preassembled GPI attached
Addition Of GPI Anchor
Vesicular Transport
• Vesicle buds off from one compartment and fuses with another• Compartments that communicate are topologically equivalent
Protein Coats In Vesicular Transport
• Cage of proteins covering cytosolic surface• Concentrates membrane proteins and deforms membrane
Clathrin Structure
• Subunits associate into triskelion• Convex framework of triskelions on cytosolic surface
• introduces curvature leading to formation of bud• linked to transmembrane cargo receptors by adaptins• removed after transport vesicle is pinched off
Clathrin coat:
Formation Of Clathrin-coated Vesicle
CGNGolgi Stack
cis cisternamedial cisternatrans cisterna
TGN
ER → CGN → cis-, medial-, trans cisternae → TGN
Organization Of Golgi Apparatus
Transporting From ER To CGN
• Exit signal on soluble cargo interacts with transmembrane receptor
• Exit signal on receptor interacts with protein coat
ER Resident Proteins
• Sorting signal for retrieval of ER proteins that enter GolgiMembrane proteins: KKXX- (COO-)Soluble proteins: KDEL- (COO-)
• Transmembrane receptor for KDEL that binds coat proteins
Golgi → ER
Processing N-linked Oligosaccharides
• Two classes formed by modifications to precursor in Golgi
Complex oligosaccharides
High-mannose oligosaccharides
Lysosomes
• Controlled digestion of macromolecules
Sorting By Recognizing M6P
• M6P added to lysosomal hydrolases in CGN• Transmembrane M6P receptors in TGN
interact with coat proteins
Specific Addition Of M6P
• Signal patch recognized by GlcNAc phosphotransferase
Lysosomal Storage Diseases• Genetic defects affecting lysosomal hydrolases• Accumulation of undigested material in lysosomes
– defective hexosaminidase A gene– accumulation of ganglioside GM2
• I-cell disease– most hydrolases missing from lysosomes– inclusion bodies– defective GlcNAc phosphotransferase gene
• Hurler’s disease– defective -L-iduronase gene
• Gaucher disease – defective glucocerebrosidase gene
• Tay-Sachs disease
Protein Sorting In TGN
• Lysosomes• Constitutive secretory pathway
– Transport vesicles from TGN to plasma membrane– Default pathway
– Sorting signal targets to special secretory vesicles• Regulated secretory pathway
• Pathways initially involve ER signal sequence, SRP
Exocytosis
• Constitutive secretory pathway: transport continually from TGN to plasma membrane
• Regulated secretory pathway: store in secretory vesicles until stimulated
Endocytosis
• Material to be ingested becomes enclosed by plasma membrane as it invaginates
• Buds off to form endocytic vesicles
• Endocytosis– Pinocytosis– Receptor-mediated
endocytosis– Phagocytosis
• Autophagy
Delivering materials to lysosomes for digestion:
Endocytic/Degradation Pathways
Receptor-mediated Endocytosis Of LDL
• Cholesterol molecules in LDL organized by protein that binds to LDL receptor
• LDL receptor interacts with clathrin-coated pit
• Mutation in LDL receptor causes familial hypercholesterolemia
Sorting In Early Endosome
• Endocytoic vesicles fuse with early endosomes
• Ligand-receptor dissociation
• Possible fates of receptor: recycling, transcytosis, degradation
Endocytic Pathway Of LDL
• LDL receptor recycled to plasma membrane• LDL degraded in lysosome to release free cholesterol
• Early endosomes form multivesicular bodies by enclosing invaginations
• Turn into late endosomes that are more acidic
• Form lysosomes by receiving hydrolases, further acidification
From Early Endosomes To Lysosomes