endomembrane system

chapter 15

internal membranes• endomembrane system

• functions as a coordinated unit• divide cytoplasm into distinct

compartments• controls exocytosis and

endocytosis• movement of molecules which

cannot pass through membrane

origins• endomembrane system

• invagination of plasma membrane• mitochondria

• endosymbiosis• chloroplasts

• endosymbiosis

transport of proteins• protein synthesis

• begins on a ribosome in cytoplasm

• sorting signals• protein transport

• nuclear transport• protein translocators• vesicular transport

pathways• endocytic

• endosomes & lysosomes• exocytic

• biosynthetic pathway• secretory pathway

• constitutive secretion• regulated secretion

• secretory granules

exocytic endocytic

golgi complex

rough ER

secretory granule

regulated secretory pathway

constitutive secretory pathway

endocytic pathway

endoplasmic reticulum• rough ER

• cisternae -• many bound ribosomes • polarity

biosynthesis in RER• synthesis of proteins

• free ribosomes• cytosolic proteins• peripheral proteins• nuclear proteins

• bound ribosomes• secretory proteins• integral proteins

Completed polypeptide

Incoming ribosomalsubunits

Start of mRNA (5′ end)

End of mRNA (3′ end)

Polyribosome

Ribosomes

mRNA

0.1 μm

Growingpolypeptides

biosynthesis in RER• bound ribosomes

• bind to ER as needed• SRP (signal recognition particle)• binds to SRP receptor

• translocon

Ribosome

mRNA

Signal peptide

SRP

1

SRP receptor protein

Translocationcomplex

ER LUMEN

2

3

45

6

Signal peptide

removed

CYTOSOL

Protein

ER membrane

biosynthesis in RER• translocon

• SRP interacts with a SRP receptor• ribosome interacts with the translocon• SRP is released

• facilitated by G-proteins

Ribosome

mRNA

Signal peptide

SRP

1

SRP receptor protein

Translocationcomplex

ER LUMEN

2

3

45

6

Signal peptide

removed

CYTOSOL

Protein

ER membrane

biosynthesis in RER• Processing

• signal sequence cleaved by signal peptidase• Carbohydrates added

• oligosaccharyltransferase• RER lumen

• chaperones assist in folding• protein disulfide isomerase adds disulfide bonds to

cysteines.

biosynthesis in RER• integral membrane proteins

• hydrophobic region generated• translocon opens laterally

• expels transmembrane segment• synthesis continues

biosynthesis in RER• glycosylation

• begins in the RER• membrane bound glycosyltransferases (GTs)

• lipid carrier (dolichol phosphate)• oligosaccharyltransferase -> transfers to polypeptide

• removal of 2 of 3 terminal glucose residues• quality control

• glycoprotein binds with calnexin (chaperone)• terminal glucose removed

• GTs recognize malfolded proteins• tag with glucose --> refolded or destroyed

• transferred to golgi

endoplasmic reticulum• smooth ER

• functions• synthesis of steroid hormones• liver processing of toxins• sequestration of Ca+

• sarcoplasmic reticulum

golgi bodies• golgi apparatus

• functionally distinct cisternae - • cis• trans

• differences in composition between cis and trans face

trans-Golgi network (TGN)

trans cisternae

medial cisternae

cis cisternae

cis- Golgi network

movement of materials• movement across Golgi

• vesicular transport model• vesicles shuttle cargo from

CGN to TGN • cisternal maturation model

• cisternae mature as they move through the apparatus

• combination?• retrograde transport via

vesiclescisternal vesicular

glycosylation• carbohydrate assembly

• glycolipids and glycoproteins• assembled across golgi by glycosyltranferases

vesicle shuttles• vesicle-mediated transport

• donor compartment• acceptor compartment• direction

• anterograde transport• toward plasma membrane

• retrograde transport• away from plasma membrane

• protein coats• two main functions

• cause membrane to curve• select components to be carried

vesicle shuttles• steps in vesicle transport

• (1) cargo selection• sorting mechanisms• coat proteins

• COPI, COPII, clathrin• bulk transport

• proteins trapped

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vesicle shuttles• steps in vesicle transport

• (2) budding• coat proteins interact with adaptor proteins

• COPII - sec23/24, sec13/31, Sar1• Sar1 - coat recruitment GTPase

COPII

adaptor protein (Sar1)

protein cargo

cargo receptor

vesicle shuttles• steps in vesicle transport

• (3) scission• scission proteins excise bud

• (4) uncoating• coat proteins detach and get reused

protein cargo

vesicle shuttles• steps in vesicle transport

• (5) transport • microtubule transport via

motor proteins• dynactin

• (6) tethering• Rab proteins bind to tether

proteins on acceptor membrane

Rab

tethering proteins

x

vesicle shuttles• steps in vesicle transport

• (7) docking• soluble NSF attachment

protein receptor• v-SNAREs• t-SNAREs

• (8) fusion • (9) disassembly

• NSF and SNAP proteins

exocytic pathway• TGN

• proteins sorted• secretory (constitutive &

regulated)• lysosomal

• constitutive secretion• plasma membrane proteins

• polarized cells - apical / basal proteins segregated in TGN

non-lysosomal proteins• regulated secretion

• aggregate as dense granules at TGN• secretory vesicles (granules)• 2 main functions

• timing, amount

release of materials• exocytosis

• triggered by increase in Ca2+ • vesicle fuses with plasma

membrane• autophagy

• digestion of organelles• autophagosome

• organelle wrapped in double membrane

• fuses with lysosome• autophagolysosome

lysosome enzyme sorting• sorting proteins at TGN

• lysosomal proteins tagged• phosphorylated mannose• recognized by MPRs

• released in clathrin-coated vesicles

lysosomes• digestive organelles

• contain acid hydrolases• digest a wide variety of biological

molecules• intracellular digestion

Spongocoel

Osculum

Pore

Epidermis Waterflow

Mesohyl

Choanocyte

Spicules

Amoebocytes

lysosomal disorders• Tay-Sachs disease

• autosomal recessive disorder• probably with multiple origins

• deficiency in hexosaminidase A• buildup of ganglioside GM2

• symptoms include• mental retardation, blindness, death

uptake of materials• phagocytosis• endocytosis

• pinocytosis• receptor-mediated

endocytosis

RME• receptor-mediated endocytosis

• clathrin coat arranged in geometric pattern

lipoproteins on cell surface clathrin polygons

on inner surface

invagination of clathrin pit

clathrin structure• clathrin

• triskelion• three heavy chains• three light chains

• binds to AP2 adaptor• binds to cargo receptors

clathrin structure• clathrin

• triskelions and adaptors geometrically arranged

• dynamin-ring• promotes pinching off of coated

pit into vesicle• uses GTP to tighten around neck

of vesicle• Hsc70 uses ATP and frees clathrin

coat

endocytic pathway• endosomes

• formed from fusion of endocytes• sorts materials arriving from plasma

membrane• early endosomes

• derived from PM vesicles• fuse with early endosome• proteins dissociate - pH affects

conformation• proton pumps lower pH of endosome• late endosome • lysosome

metabolizing cholesterol• cholesterol packaging

• lipoproteins • LDLs and HDLs

• LDLs taken up by RME• lysosomes • release cholesterol for use

endothelial injurymacrophage

fibrous cap

engulfing particulate• phagocytosis

• engulfing large particles• phagosome

• fuses with lysosome• phagolysosome

engulfing particulate• phagocytosis

• cells of immune system• engulf foreign material• some pathogens can

withstand being phagocytized• Mycobacterium

tuberculosis• Coxiella burnetti• Listeria

monocytogenes

plant cell vacuoles• vacuoles

• aqueous storage organelles• tonoplast - water vacuole in plants

• contains active transport system for K+ ions • aquaporins