Vesicles and Secretion 2

Clathrin-coated vesicles

Clathrin-coated vesicles are the most common type released from the trans-Golgi, and consist of two types of layers: clathrin triskelions on the outside and adapter protein complexes on the inside.

AP complexes

AP complexes 1-3 consist of 4 subunits + GGA (AP with only one subunit)

ARF small GTPase required for assembly of all four complexes

Different APs bind to sorting signals in different cargo proteins in the proteins in the membrane of the vesicle, and are used for different transport routes.

Which are the three main types of vesicle coats and in

COPI - retrograde ER - Golgi

COPII - andrograde ER - Golgi

Clathrin - trans-Golgi to destination

What is mannose-6-phosphate used for during vesicle trafficking?

Uncoating is done by Hsp70, and exposes v-SNAREs for binding to target membranes. There are two paths, either directly with the AP3 complex or via late endosomes with clathrin and AP complexes. Lysosomal proteins carry specific modifcations generated in the ER, and targeting to late endosomes require phosphorylation of carbon 6 of mannose in the cis-Golgi before it’s recognized for transport.

M6P receptors bind M6P in the trans-Golgi, and vesicles with M6P modified proteins fuse with late endosomes before the M6P proteins are released due to the lower pH in the late endosomes. The late endosome fuse with lysosomes, and the M6P-receptor is recycled to the trans Golgi.

What is the difference between a protein and a proprotein?

Some enzymes are synthesized as proproteins, but only become functional after they have left the Golgi and are cleaved by endoproteases.

What is a clathrin-coated pit?

A clathrin-coated pit is a specialized patch of the plasma membrane with special receptors used during endocytosis. Lipids are taken up as lipoprotein particles, and the LDL receptor has a binding site for AP2 in the cytoplasmic domain. After shredding the coat, the early endosome fuses with a late endosome, and the LDL particle is released from the receptor before the late endosome fuses with a lysosome. V-class proton pumps move protons into endosomes and lysosomes, and clorine anions need to enter the lumen to prevent build-up of an electric potential. LDL particles bind to the ligand binding arm of the LDL receptor, and histidines in the beta-propeller domain of the LDL receptor are protonated and bind to the ligand-binding arm at low pH.

How are multivesicular endosomes formed?

The membrane of late endosomes contain proteins that are required for lysosomal function and proteins to be degraded, and membrane proteins to be degraded are monoubiquitinated in their cytosolic domain. The monoubiquitinated proteins recruit ESCRT proteins which cause vesicle formation into the lumen to form multivesicular endosomes.

What is the function of autophagy?

Autophagy recycles molecules and organelles! Autophagosomes originate from the Golgi and/or from the ER. They start as a membrane-bounded structure that becomes cup-shaped and finally encloses some cytosol or an organelle. Ubiquitination is a signal for enclosing organelles.The autophagosome grows by fusion with smaller vesicles that contain Atg8, and Atg8 is removed from the membrane when the autophagosome is completed before it fuses with a lysosome.