BIOL-3510: Ch. 12 Intracellular Transport

Vesicular transport

Extends outward from the ER to the plasma membrane AND inward from the plasma membrane

Exocytosis

cytosolic particles moving outward

Endocytosis

outward particles moving into the cytosol

Include phagocytosis and pinocytosis

Transport vesicles

Carry membrane components between compartments of the endomembrane system and plasma membrane via budding from one membrane and fusing with the next

Secretory pathway

Also called the exocytic pathway

Moving inward OUT

Endocytic pathway

Moving outward IN

Vesicle budding is driven by:

Assembly of a protein coat on the cytosolic surface

Clathrin-coated vesicles

Transport selected cargo molecules

Adaptins

Capture cargo molecules and secure the clathrin coat to the vesicle membrane

Dynamin

GTP binding protein that construct the separation of the vesicle from the membrane

SNARES

Direct transport vesicles to their targer membranes by forming a coiled coil between v-SNAREs and t-SNAREs

v-SNAREs

Found on the surface of the transport vesicle

t-SNAREs

Found on the surface of the target membrane

RAB

Acts as a ‘zipcode’ on the vesicle membrane that tethering protein binds to

The secretory pathway starts in:

The ER, where proteins are covalently modified

Disulfide bonds

Form in the ER to stabilize protein structure

Glycosylation

How sugars are added to proteins

Exit from the ER is:

Selective; proteins that do not fold/assemble properly are retained in the ER by chaperone proteins

If chaperones in the ER fail:

Misfolded proteins are exported to cytosol and degraded by the proteasome

In the Golgi apparatus:

Proteins are further modified and sorted

O-linked glycosylation

Sugars are added to oxygen atoms in serine or threonine

trans-Golgi network to plasma membrane

Secretory proteins are released from the cell by exocytosis

Phagocytosis

Cell eating

Specialized phagocytic cells ingest large particles

Pinocytosis

Cell drinking

Fluid and macromolecules are taken up

Receptor-mediated endocytosis

Provides a specific route into animal cells

Endosomes

Sorting site for endocytosed molecules

How do influenza and HIV enter the cell?

Via receptor-mediated endocytosis

Lysosomes

Membranous sacs of hydrolytic enzymes that are the principle sites of intracellular digestion

Lysomonal membrane contains:

proteins to transport the products of digestion back to the cytosol

H+-ATPase, which maintains acidic pH