Chapter 13 Intracellular Vesicular Traffic

Vesicular transport

The endocytic and biosynthetic-secretory pathways

The three well-characterized types of coated vesicles differ in their coat proteins

Utilization of different coats in vesicular traffic

Assembly of a clathrin coat drives vesicle formation Clathrin-coated pits and vesicles

The structure of a clathrin coat

The assembly and disassembly of a clathrin coat The pinching-off and uncoating of coated vesicles are regulated processes The assembly and disassembly of a clathrin coat

Not all coats form basketlike structures A model for retromer assembly on endosomal membranes

Phosphoinositides mark organelles and membrane domains

The intracellular location of phosphoinositides

leaflets of the lipid bilayers flow together The role of dynamin in pinching off clathrin-coated vesicles from the membrane Dynamin together with other proteins destabilize the membrane so that the noncytoplasmic leaflets of the lipid bilayers flow together

The coats of COPI and COPII vesicles consists of large protein complexes that are composed of - seven individual coat-protein subunits for COPI coats, and - four individual coat-protein subunits for COPII coats Some COPI coat-protein subunits show sequence similarity to adaptins

Vesicular transport does not necessarily occur only through uniformly sized spherical vesicles, but can involve larger portions of a donor compartment

Monomeric GTPases control coat assembly Assembly of a COPII-coated vesicle

Rab proteins guide vesicle targeting

SNARE proteins and targeting GTPases guide membrane transport SNAREs contribute to the selectivity of transport-vesicle docking and fusion, while the GTPases (Rabs) regulate the initial docking and tethering of the vesicle to the target membrane

The formation of a Rab5 domain on the endosome membrane

SNAREs mediate membrane fusion Structure of a trans-SNARE complex

A model for how SNARE proteins may ccatalyze membrane fusion

Interacting SNAREs need to be pried apart before they can function again

Viral fusion proteins and SNAREs may use similar fusion mechanisms The entry of enveloped viruses into cells

Proteins leave the ER in COPII-coated transport vesicles

Only proteins that are properly folded and assembled can leave the ER

Homotypic membrane fusion Transport from the ER to the Golgi apparatus is mediated by vesicular tubular clusters Homotypic membrane fusion

Vesicular tubular clusters move along the microtubules to carry proteins from the ER to the Golgi apparatus

The retrieval pathway to the ER uses sorting signals The best-characterized signal for ER membrane proteins is KKXX present at the C-terminus and this signal interacts directly with the COPI coat . Soluble ER resident proteins contain the signal sequence KDEL at their C-terminus and this signal interacts with the KDEL receptor, a multipass transmembrane protein.

cluster and the Golgi apparatus The pH-dependent retrieval of ER resident proteins from the vesicular tubular cluster and the Golgi apparatus Many proteins are retained in the ER as a result of kin recognition

The length of the transmembrane region of Golgi enzymes determines their location in the cell

The Golgi apparatus consists of an ordered series of compartments

Molecular compartmentalization of the Golgi apparatus

Oligosaccharide chains are processed in the Golgi apparatus

The two main classes of N-linked oligosaccharides found in mature mammalian glycoproteins

Oligosaccharide processing in the ER and the Golgi apparatus

Proteoglycans are assembled in the Golgi apparatus

Two possible models for transport of proteins through the Golgi apparatus

Lysosomes are the principal sites of intracellular digestion

A model for lysososme maturation

Multiple pathways deliver materials to lysosomes

A mannose 6-phosphate receptor recognizes lysosomal proteins in the trans Golgi network

The M6P receptor shuttles between specific membranes

A signal patch in the hydrolase provides the cue for M6P addition

Endocytosis Two main types of endocytosis distinguished on the basis of the size of the endocytic vesicle formed. Phagocytosis involves ingestion of large particles and is a triggered process. Pinocytosis involves ingestion of fluid and solutes via small pinocytic vesicles and is a constitutive process that occurs continuously

Specialized phagocytic cells can ingest large particles A macrophage ingesting red blood cells

Phagocytosis by a neutrophil

Pinocytic vesicles form from coated pits in the plasma membrane Formation of clathrin-coated vesicles from the plasma membrane

Not all pinocytic vesicles are clathrin-coated Caveolae in the plasma membrane of a fibroblast

Cells import selected extracellular macromolecules by receptor-mediated endocytosis Most cholesterol is transported in the blood as low-density lipoprotein (LDL) particles

Normal and mutant LDL receptors

The receptor-mediated endocytosis of LDL

Possible fates for transmembrane receptor proteins that have been endocytosed

Exocytosis

Constitutive secretory pathway – all cells Regulated secretory pathway – found mainly in specialized cells

The three best-understood pathways of protein sorting in the trans Golgi network

Secretory vesicles bud from the trans Golgi network

Exocytosis of secretory vesicles

Proteins are often proteolytically processed during the formation of secretory vesicles Alternative processing pathways for the prohormone proopiomelanocortin

Polarized cells

Sorting of plasma membrane proteins in a polarized epithelial cell

Most synaptic vesicles are generated by local recycling from the plasma membrane in the nerve terminals

A model of a synaptic vesicle