CONTACT SITES Flashcards
There can be transfer of material where there is no fusion between membranes. What is this called?
Membrane contact site.
When were membrane contact sites identified?
1957 by Porter and Palade
EM shows electron dense sites where there is a higher conentration of protein. Tend to be ovbserved in neurons and muscle cells.
Indicates that it might be involved in Calcium homeostasis also.
What is STIM 1?
It is a calcium sensor in the ER membrane.
After muscle contraction when calcium stores are depleted what happens molecularly?
Calcium depletion causes a conformational change in the STIM1 protein and it becomes more positvely charged. This allows the protein to interact with the negative charges of PIP2 on the plasma membrane. When STIM is in close proximity to PIP2, it can interact with an ORAI chanel, a SOCC. Calcium can no enter the cell via the ORAI channel. Calcium can no be transferred directly back into the ER lumen.
There is no fusion between membrane and there is no vesicle formation.
The ER makes contact with many different organelles in the cell. What is another known example?
Mitochondria.
What has made it easier to understand structure and function of this phenomena?
Greater resolution electron microscopy has allowed us to understand the environment of membrane contact sites, and the distance between membranes.
It has also been shown that membrane contact sites form in parts of the ER where ribosomes are not present.
Live imaging has allowed us to see the dynamics of this phenomena. By transfecting cells with flourescent tagging, we can follow a cells behaviour and the time course.
How does the ER behave around mitochondria?
The ER encircles mitochondria which are constantly undergoing fission which is important for homeostasis and cell death. The ER establishes the site of excision of the mitochondria.
The cytoskeleton of the ER contributes to the position of the organelles.
What is the general structure of a membrane contact site?
Always part of the smooth ER (no ribosomes)
Membrane become very close (3-15nm apart)
They are long lived and take time.
Protein components often extend to allow bridges to from between the ER and target organelle accounting for the gap between membranes, these make protein tethers.
How can proteins tethering be visualised?
Through live cell imaging of membrane contact sites you can see the ER and the tethers to the organelles.
What are ER endosome contacts important for?
Endocytic pathways such as lipid transfer.
Eg LDL
What possible mutation can a patient have when they have high cholesterol?
Mutations in the clathrin mediated pathway.
Endosomes can drag sections of the ER. How and why?
As an endosome moves through the cell, it can pull ER tubules along with it. This might have a role in positioning of organelles within a cell.
30% of all cholesterol is take up by the cell and delivered by the endosomal pahway in a unidirectional manner. What is this mediated by?
Membrane contact sites.
What proteins are important in the endosome for delivery of cholesterol to the ER?
NPC1 and NPC2
What does NPC1 do?
Facilitates the movement of cholesterol out of the liposome to the ER.
