Membrane trafficking (revision)

When the cell wants to move things around it typically pakages them up into a vesicle and transports the vesicle. This is more efficient than transporting the molecules one by one, though it requires a more elaborate system (marking vesicles on where they need to go). 

Typically proteins are tagged by a signal sequence on the N-terminal which puts them into a specific vesicle. Lipids that need to be transported make up the wall of the vesicle. The vesicles are tagged with a specific label. 

A tricky process to master because the more specific that labelling the more throughly and careful the checking needs to be, which is of energetically costly to the cell and is slower. 

Sorting in membrane trafficking is quite relaxedbut gets reviewed multiple times and if for example something destined for the medial-Golgi goes too far it is transported back again. 

Everything in this process is recycled and most of the tagging is done by proteins but lipids can be involved. 

Almost all the movement is along microtubule tracks of the cytoskeleton and uses motor proteins to carry the vesicles. This makes the process directional and very organised. 

The process of ligand-mediated endocytosis was brought about by membrane rafts. 

• Ligand binds to a receptor and activates it 
• Activation of receptor leads to formation of a membrane raft
• Proteins, caveolin, bind to the membrane raft and insert halfway into one membrane leaflet and curve  the membrane
• Caveolin recruits more proteins (scaffolding proteins cavin and clathrin) which make a coat around the caveolae. 
• Caveolae are pinched off at the top and move into the cell. 

The whole system is self assembling