Vesicular Transport Pages

The general flow of membrane enclosed material within cells

15_17_Vesicles_bud.jpg The general flow of membrane enclosed material within cells - utilizes the endomembrane system as a system of highways

Vesicular Transport Question: –If you were the designer of the cell how would you make sure that different components were delivered to the correct locations? –You could use labels - but the cell cannot type! –You could use sounds - but the cell does not speak! –How then!

15_18_Clathrin_EM.jpg Answer: use special protein paint to coat each vesicle on cytoplasmic side. Each paint color directs its vesicle along just one pathway The cell does the same using different combinations of proteins to cover each vesicle.

Vesicle formation is driven by proteins & energy General Properties of vesicle formation A number of proteins have a role in the formation of each type of vesicle. These vesicles are thus called coated vesicles. Utilizes energy from ATP or GTP The coat is shred soon after vesicle formation when its job is complete. It serves two important functions: –1) Shapes the vesicle –2) Captures material for transport

15_19_Clathrin_vesicle.jpg Best studied system are vesicle coats made of clathrin. These vesicles bud from either the Golgi or the plasma membrane. Interplay of several proteins - 1.Cargo receptors 2.Adaptin 3.Clathrin, and 4.Dynamin The material itself carries transport signals, that binds to cargo receptors. It appears that there are different kinds of adaptins in different areas

SNAREs Vesicles travel along the cytoskeleton highway of the cell - more about these in a latter lecture How does one get a vesicle to fuse with its target membrane - when it arrives at its destination? ANSWER: use special complementary proteins on both the vesicle and the target membrane SNAREs - –vSNAREs on the vesicle & –tSNAREs on the target membrane These proteins bring the membranes close enough together to fuse - uses energy Each destination seems to have its own unique set of SNAREs

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15_21_membr_fusion.jpg The SNARE proteins act like bag ties. They twist around each other bringing the two membranes close together until they fuse!

Secretory Pathways Cells have a need to move material to the cell surface - they use a mechanism called exocytosis to move material from the ER to the plasma membrane via the Golgi The material is often chemically modified, by way of glycosylation or disulphide bonds These sugars are added by way of special lipids that are present just within the lumen of the ER that hold these sugar entities… –Dolichol is a good example of such a carrier molecule

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15_23_Chaperones.jpg Material leaving the ER is stringently checked for quality - a quality control step. Material that fails to pass these tests is held back and eventually degraded. Some material actually carries a ER retention signal and if it is not properly removed the material is returned to the ER from the Golgi!

15_24_Golgi.jpg The Golgi apparatus is normally located near the nucleus. It consists of a collection of flattered membranous disks. It has orientation - cis face, trans face The Golgi itself may modify material too!

Secretory Pathways Every cell needs to get material to the surface for some reason - adding new membrane before dividing, placing cell receptors on the cell surface, replacing the sugar coat on the cell surface. Every cell uses the constitutive exocytosis pathway to achieve this task. Other cells have a special function of secreting material - they have an added pathway - regulated exocytosis pathway

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15_29_Secretory_vesicl.jpg A classic example of vesicles that excrete material - in this case these cells are pancreatic B cells releasing insulin into bloodstream

Endocytic Pathays Endocytic pathways bring material into the cell in a process know as endocytosis Two types exist –1) Pinocytosis - means ‘cellular drinking’ –2) Phagocytosis - means ‘cellular eating’ Phagocytosis Ameoba was first noted to inject its food via phagocytosis. Macrophages of the immune system ingest bacterial pathogens in this manner too…

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15_31_macrophage.jpg A macrophage ingesting, via phagocytosis, two RBC. FACT: each day 10E11 RBC are deleted from your body by this process!

Pinocytosis A constant ongoing process where small vesicles are continuously pinched off the plasma membrane and internalized A typical macrophage will pinocytose an equivalent of its entire membrane area in about an hour! There is a balance between endocytosis and exocytosis to maintain the cell volume This process is usually performed by the clathrin- vesicle coated pits and leads to the formation of endosomes - organelles destined for degradation pathways

More specialized examples… Pinocytosis is indiscriminate - anything in the vacinity of the vesicle is internalized. Some cells have evolved plasma membrane receptors that will bind to and concentrate certain macromolecules that the cell needs to take up This is known as receptor-mediated endocytosis Can concentrate the material taken up by a factor of 1000 fold or more. Requires special receptors - great example is LDL receptors on cell surfaces…

15_32_LDL_enters.jpg The cell gains its cholesterol via specialized receptors. Note that the receptors are recycled to the cell surface!

15_33_type_of_receptor.jpg The endomomes are key players in the endocytosis pathway, just as the Golgi was in the exocytosis pathway. Endosomes determine the eventual outcome for ingested material There are three possible outcomes from an early endosome: 1)Recycling 2)Degradation 3)Transcytosis

15_34_lysosome.jpg Lysosomes have harsh environments designed to denature and degrade Not know how lysosomes form: they may convert from an endosome or be made directly from the Golgi

15_35_path_lysosome.jpg The different cellular highways bring material to the lysosome