One of the important things to remember about cells is that they are dynamic. If you look at a cell under a live microscope you'll see areas which are constantly moving.
Much of this movement is to regulate the contents of the cell. This includes bringing material into the cell from the extracellular environment or the plasma membrane (endocytosis) or releasing material made within the the cell to the outside (exocytosis).
Endocytosis is primarily involved in removing receptors from the cell surface and taking them to be destroyed. This is an important process because it ends the signal from the receptor. The stages of endocytosis are helpfully explained in the figures below, but here's a summary:
Most receptors will follow either the recycling pathway or the degradation pathway, although not in 100% of cases. An example of a receptor which is usually recycled is the Transferrin Receptor (TfR). A receptor which is usually degraded is the Epidermal Growth Factor Receptor (EGFR).
There could be a link between endocytosis and cancer. Some receptors, such as the EGFR, are responsible for transducing (converting) the signal from growth factors. As the name suggests, these factors trigger cell growth. Endocytosis is responsible for ending the signal (also known as signal attenuation) by removing the ligand and destroying the receptor. If the signal is not stopped, the cell will be told to grow indefinitely. This could lead to cancer, which occurs when cells grow and divide out of control.
The endocytic machinery is also used by certain viruses such as human immunodeficiency virus (HIV). These viruses hijack the ESCRT proteins and use them for their own nefarious purposes! The virus uses the ESCRTs to help release copies of itself from the infected (host) cell, and these copies will go on to infect neighbouring cells.
Exocytosis is (unsurprisingly) the opposite of endocytosis. This is when material which has been made inside the cell is released into the extracellular environment.
Many proteins are synthesised in the cell cytoplasm. However, if they are destined to be released from the cell, they may pass through the secretory pathway. These proteins are made at the membrane of the rough endoplasmic reticulum, so called as it has the protein-making ribosomes attached to its surface.
The protein will then be packaged into a vesicle and delivered to the Golgi apparatus, which can make several modifications to the protein along the way.
After passing through the Golgi apparatus, the protein may be enclosed within another vesicle which pinches off and travels towards the plasma membrane. This vesicle then docks and fuses with the plasma membrane with the aid of a group of proteins called SNARE complexes. This will involve a specifc SNARE complex on the vesicle (called a v-SNARE) binding to a specific SNARE complex on the plasma membrane (called the t-SNARE or "target SNARE"). These SNARES are highly specific to allow correct delivery of the contents to the right compartment. Different combinations of v- and t- SNARE complexes are responsible for targeting different vesicles to a specific cell compartment - e.g. the Golgi to the plasma membrane in exocytosis, or the endoplasmic reticulum to the Golgi.
Another set of proteins called Rab proteins are thought to help to make sure the vesicle docking is specific. Certain Rab members are specific to each compartment membrane, e.g. Rab5A is found on the plasma membrane, Rab2 is found on the Golgi. Rab proteins are GTPases, meaning they require the nucleotide GTP to activate them.
Once the vesicle has fused to the plasma membrane, the cell can then release its content to the extracellular environment.
Alberts et al Molecular Biology of the Cell p.721-723, p.749-756. 4th Edition (2002) Garland Science.
Piper and Katzmann (2007) Biogenesis and Function of Multivesicular Bodies Annu. Rev. Cell Dev. Biol. 23:519–47
Mosesson et al (2008) Derailed Endocytosis: an emerging feature of cancer Nat Rev Cancer 8(11):835-50.
http://www.ncbi.nlm.nih.gov/books/NBK21471/ Overview of the Secretory Pathway, fromLodish H et al, Molecular Cell Biology. 4th edition (2000), W. H. Freeman
Schiavo et al (1997) Binding of the synaptic vesicle v-SNARE, synaptotagmin, to the plasma membrane t-SNARE, SNAP25, can explain docked vesicles at neurotoxin-treated synapses Proc. Natl. Acad. Sci. USA 94:997–1001
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