Endocytosis & Exocytosis

By : Matt Sargeant

What is Endocytosis and Exocytosis?

• Endocytosis is a process by which cells absorb molecules (such as proteins) by engulfing them. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane.

• Exocytosis - is the durable process by which a cell directs the contents of secretory vesicles out of the cell membrane. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane.

Endocytosis• In endocytosis, the cell engulfs some of its

extracellular fluid (ECF) including material dissolved or suspended in it.

• A portion of the plasma membrane is invaginated and pinched off forming a membrane-bounded vesicles called an endosomes.

Types of Endocytosis

• Phagocytosis- When the cell engulfs particulate material, such as bacterium or fragments of organic matter.(Cell Eating)

• Pinocytosis – When the cell engulfs something that is liquid.(Cell Drinking)

• Receptor Mediated endocytosis – When receptors on the cell membrane bind and then enters the cell, such as with LDL(Low Density Lipoprotein).

Receptor Mediated Endocytosis (RME)• 1. Cargo molecules attach to the receptor

proteins.• 2. The adaptin attaches to the receptor

proteins.• 3. Clathrin attaches to the adaptin causing

invagination.• 4. Dynamin pinches off vesicle into the

cytosol.• 5. Clathrin coat falls off.• 6. Travels along microtubules with motor

protein towards the negative end to it’s destination. (towards the positive for exocytosis)

Dynamin

• a large GTPase involved in the scission of nascent vesicles from parent membranes.

• http://www.endocytosis.org/Dynamin/Dynamin_pinchase.mov

Exocytosis

• The means by which molecules such as proteins, hormones, and waste are secreted from the cell.

synaptotagins• The synaptotagmins are a family

of proteins with an N-terminal transmembrane domain and two cytoplasmic C2 domains. Many C2 domains mediate calcium-dependent binding to negatively charged membranes. Synaptotagmin-1 is localized to synaptic vesicles and is the trigger for their calcium-induced exocytosis. The two C2 domains of synaptotagmin-1 insert into the membrane upon calcium binding.

Snare proteins• SNARE proteins contain

motifs that fold into a stable 4 helix bundle pulling the vesicle into close proximity with the plasma membrane

• 1. Vesicle meets cognate membrane

• 2. SNAREs engage• http://

www.endocytosis.org/Synaptotagmin/SNARE_zippering.mov

Snare Engagement

• 3. SNARE complexes form, and with synaptotagmin membranes undergo fusion via a high curvature intermediate

• Most intracellular membrane fusion events are SNARE-dependent

Citations and references• http://en.wikipedia.org/wiki/Endocytosis• http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/

dl/free/0072437316/120068/bio02.swf::Endocytosis%20and%20Exocytosis• • http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Endocytosis.html• • Snare video• http://www.endocytosis.org/Synaptotagmin/SNARE_zippering.mov• snare page• http://www.endocytosis.org/Synaptotagmin/• • dynamin animation• http://www.endocytosis.org/Dynamin/Dynamin_pinchase.mov• • http://www.annualreviews.org/doi/full/10.1146/annurev.biochem.78.081307.11054

0• http://en.wikipedia.org/wiki/Exocytosis