Protein Sorting and Transport

Gene Expression

DNA→RNA→Protein

Transcription in nucleus

Translation on cytosolic ribosomes

ER receives translated proteins → folded/processed → transported to Golgi in vesicles → shipped to destination

Endoplasmic Reticulum

Rough ER- covered by ribosomes on cystosolic face for protein processing

Smooth ER- no ribosomes, functions in lipid metabolism

Cytosolic ribosomes

Binds to mRNA to form poly ribosome

Goes to nucleus, mitochondria, chloroplasts, peroxide meds, cytosol, peroxisomes

ER Ribosomes

Binds to mRNA to form poly ribosomes

Ribosomes dock to ER Membrane and incorporates into ER Vesicle → transports to Golgi, plasma membrane, nuclear membrane, peroxisome membrane, lysosomes, secretory vesicles

Secretory Proteins

Ribosomes targeted to go to rER - ERLS (on N terminus) attaches to Signal Recognition Particle (SRP) in cytosol and SRP Receptor in ER membrane —> Passes to Sec61/SecY translocon in membrane to enter ER —> bound for secretion

Chaperone Proteins (BiP)

Assists in correct folding into 3D conformation as the chain enters ER Lumen through translocon

Disulfide bonds

Proteins in rER meant for PM, lysosomes, and endosomes form disulfide bonds

Protein Disulfide Isomerase (PDI)

Oxidizing environment of rER uses this to form disulfide bonds for proteins meant for PM, lysosomes, and endosomes

Glycosylation

Most other proteins are converted to glycoproteins via glycosylating enzymes to

• Protect from degradation

• Keep in rER until folded

• Serve as transport signal for vesicles

• Serve as cell-cell recognition molecules (glycocalyx)

Oligosaccharyl transferase

Catalyzes oligosaccharide from monomers in glycosylation

Target aa (asparagine)

Dictates where oligosaccharide is added on protein

GPI Anchors

Anchors plasma membrane proteins. C Terminal sequence anchors in membrane and enzyme cleaves and attaches it to GPI anchor

ER Retention Signal

Ensures proteins meant to be in ER stay or escapees are returned

Proteosome

If folding/assembly of protein by chaperone protein fail, proteins are exported to cytosol for degradation by proteosome

Unfolded Protein Complex (UPR)

Misfolded proteins accumulate → UPR → Produces more ER/Chaperones or apoptosis

IRE1 Pathway/ ATF6 Pathway/ PERK Pathway

Sensor proteins that detect unfolded proteins to restore balance

Smooth ER

Lipids synthesizes on cytosolic membrane face of sER

Flippases catalyze transfer of lipids to other side of bilayer

Cholesterol and ceramide synthesis

Golgi Apparatus

Network of interconnected, membranous sacs

Receive/process/sort proteins

Synthesize glycolipids and sphingomyelin

N-linked Glycosylation → Glycoproteins

Modification of glycoproteins by N-linked oligosaccharide addition to N on asparagine and mannose removal in rER

Lysosomal Protein Glycosylation

Mannose proteins are phosphorylates and recognized by receptor in trans-Golgi → directs proteins to lysosomes

O-linked Glycosylation → Proteoglycans

Adding single sugars to oxygen of serine and threonine side chains to from proteoglycans → crucial to extracellular matrix

Proteoglycans → have GAG chains that are modified by sulfate groups

Lipid/Polysaccharide Metabolism

Ceramide and cholesterol made in sER are transported to Golgi to make sphingomyelin and glycolipids

Vesicular Transport

Vesicles bud off to travel to endosomes and lysosomes

Secretory granules mature to fuse with PM and release

Cytosolic Coat

Transport vesicles have coat that sheds once ready to fuse with designated organelle

Clathrin: mediates transport back and forth from trans-Golgi to destinations (endosomes,lysosomes,PM)

COPI: ER Golgi Intermediate Compartment to ER

COPII: ER → ERGIC → Golgi

Adaptins

GTPases recruit adaptins to donor membrane. Adaptor proteins trap cargo receptors and cargo molecules and assemble clathrin coat

Rab Proteins/Tethering Proteins

Transport vesicles must recognize target to unload cargo

Rab Proteins (on surface of vesicles) recognize tethering proteins (on target) → coat removed

V-SNAREs interact with t-SNAREs for membrane fusion

Lysosomes

Membrane-bound organelles that contain digestive enzymes to break down macromolecules

Mutations in enzymes can lead to lysosomal storage disease (gaucher disease)

Digestive enzymes are active at pH 5 maintained by active transport (ATP hydrolysis energy) proton pumps in lysosomal membrane

Role of Lysosomes

Digest materials ingested during endocytosis

Transport vesicles from trans-Golgi fuse with late endosomes → mature lysosomes

Ingested materials in endosomes hydrolyzed by lysosome → recycled or exocytosed

Autophagy

Gradual degradation of old cell components; stress response during starvation to rid of nonessential components

Old organelle or PM section enclosed by ER-derived membrane (autophagosome made) → lysosome fuses with autophagosome (phagolysosome made) → organelle broken down by enzymes