Protein Trafficking​ & Virion Assembly​

Where are “free” ribosomes found

in the cytosol (not attached to the ER)

Where do proteins made by free ribosomes go

Stay inside the cell — cytosol, nucleus, mitochondria

What are soluble cytosolic proteins

Proteins that remain in the cytosol and move by diffusion or binding partners

What are peripheral membrane proteins

Proteins that attach to the outside of membranes using binding partners and diffusion

What are lipid-anchored proteins (cytosolic side)

Proteins held to membranes by lipid tails, moving by 2D diffusion or lipid rafts

What are nuclear proteins

Proteins that enter the nucleus using import/export machinery

What are mitochondrial proteins

Proteins imported into mitochondria post-translationally using targeting sequences.

Where are rough ER ribosomes located

Attached to the endoplasmic reticulum (ER)

What pathway do these proteins enter

The secretory pathway (ER → Golgi → final destination)

Name four types of proteins made on the rough ER

• Integral membrane proteins — built into membranes.

• Secreted proteins — exported outside the cell.

• Lumenal proteins — remain inside ER, Golgi, or lysosomes.

• Lipid-anchored proteins — attached to membranes by lipid tails.

What is a post-translational modification

A chemical change made after translation that affects localization or function

What is prenylation

Addition of a hydrophobic prenyl (fatty) group to a cysteine at the C-terminus (CAAX box)

What does prenylation do

Anchors proteins to the inner leaflet of the plasma membrane

what is myristoylation and what does it do 

Addition of a 14-carbon fatty acid (myristoyl group) to the N-terminal glycine; Weakly attaches proteins to the inner leaflet of the plasma membrane

What is a GPI anchor

A sugar-lipid added to the C-terminus to tether proteins to the outer leaflet of the membrane

Where are GPI-anchored proteins found

On the cell surface, often as enzymes or receptors

What is an NLS

Nuclear Localization Signal — directs proteins into the nucleus

Describe the nuclear import process.

• Importin binds NLS in cytosol.

• Complex recognized by nuclear pore complex (NPC).

• Protein transported into nucleus.

• Ran–GTP binds importin → releases cargo.

• Importin recycled back to cytosol

What is an NES

Nuclear Export Signal — directs proteins out of the nucleus.

Describe nuclear export.

• Exportin binds NES inside nucleus.

• Ran–GTP binds to form exportin–cargo complex.

• NPC transports it to cytoplasm.

• Ran–GTP hydrolyzed → releases cargo.

• Exportin and Ran–GDP recycled.

What is the nuclear pore complex (NPC) made of

: ~30 proteins called nucleoporins (Nups)

What passes freely through NPC

Molecules <40 kDa by diffusion

What requires active transport

Molecules >40 kDa using importin/exportin and energy

What is post-translational import

Protein made first, then imported

What is clathrin

A coat protein that helps form vesicles during endocytosis

What is a triskelion

Three clathrin heavy + three light chains forming a three-legged structure

What is the function of clathrin

Drives vesicle budding and curvature; coat removed after budding

What do adaptor proteins (adaptins) do

Link clathrin to membrane receptors to ensure correct cargo selection

What is receptor-mediated endocytosis

A selective process using receptors, adaptors, and clathrin to import specific molecules

What are SNAREs

Membrane proteins that mediate vesicle–target membrane fusion

Types of SNAREs

V-snare T-snare

How does fusion occur

v-SNARE binds t-SNARE → forms complex → pulls membranes together → fusion releases contents

principal ER modifications.

• Disulfide bond formation (stability).

• Proper folding (BiP chaperones).

• Proteolytic cleavage of signal sequences.

• Assembly into multimeric complexes.

• Glycosylation (addition of sugars)

What is an oligosaccharide

A short sugar chain added to proteins for folding, stability, or signaling

Describe O-linked glycosylation

Sugar added to Ser/Thr hydroxyl groups; short chains; common in secreted/surface proteins.

Describe N-linked glycosylation

Sugar added to Asn amide; long/branched chains; starts in ER, continues in Golgi; aids folding & quality control