L11 - inter-membrane contact sites Flashcards
what are membrane contact sites? (MCS)
close appositions between 2 organelles
Are there membrane contact sites between the endoplasmic reticulum and the mitochondria?
- yes
- ER holds the mitochondria like a hand - membrane become very close but do not fuse
What does the small lipid anchor have to do with MCS?
- lipid is embedded into the membrane which allows organelles to have affinity/bind to organelles which they are making contact with
How can protien tethers (lipid anchors) and bridges be seen between MCS?
- by electon microscopy
Give some of the functions of MCS?
-provide a membrane platform for
- signalling
- non-vesicular lipid exchange
- mobilization of calcium stores
what organelle is the major calcium store of cells?
ER is major calcium store of cells
- close bringing together of calcium stores via intermembrane contact sites
give the major adaptor protein in mammalian cells
AP2
adaptor protein 2
Give the fundamental mechanisms of (clathrin dependent) endocytosis
- ligand binds to transmembrane receptor
- ligand-receptor complexes become clustered in clathrin coated pits
- adaptor protein acts as a link between cargo in the vesicle and vesicle coat (clathrin)
- membrane pinched off by dynamin and early endosome forms containing cargo
what do rab proteins do?
- key in sorting of the early endosome
what rab proteins are abundant on early endosomes?
rab 4
rab5
give some clathrin - independent endocytic pathways
- non-coated vesicles
- calveolae
- macropinocytosis
- phagocytosis
what are calveolae?
- cup shaped invagination of membranes
- invaginations form rosettes in muscle cells - store
- rosettes are endocytotic - take up cargo
what happens in macropinocytosis?
- non-selective form of endocytosis where cells take up the EC environment
- immune cells use this to sample environment they are in
-mobilizaiton of actin forms a pseudopod which can take up EC en. - cells ruffle
what does phagocytosis take up?
debris and pathogens
what does clathrin dependent endocytosis take up?
- nutrience
- signalling receptors
How many subunits do adaptor proteins have?
- 4 subunits
which adaptor protein subunit does clathrin bind to?
Beta subunit of adaptor proteins
which adaptor proteins bind to clathrin?
AP1 (trans-golgi-network)
AP2 (plasma membane)
how many adaptor proteins are there?
AP1-5
where is AP5 found?
adaptor protein found on late endosomes (still require sorting)
which subunit of adaptor proteins recognises sorting signals?
Mu (μ)
- cargo binding so recognises sorting signals on cargo
what is co-incidence detection?
- where adaptor proteins bind to cargo and bind to phosphoinositides
(both are required which allows co-localisaiton of adaptor proteins)
-needed as mu subunits recognise similar signals and same cargo/sorting signal may be present in multiple locations.
what phosphoinositides do AP1 and AP2 bind to?
- AP1 binds to cargo and PI4P
- AP2 binds cargo and to PI(4,5)P2
what helps to define the endocytotic intermediates?
rabs and phosphoinositides
different PIs at different places in the cell- lable with GFP to visualise localisations
what are CLASPs?
clathrin associated sorting proteins
what are the recognition motifs on Adaptor proteins?
WxxF and DP[FW]
what is the dynamin homolgue in drosophila?
shibre
(GTPase - riquired for pinching off of clathrin coated vesicles)
- accumulation of endocytotic intermediates if dynamin cannot function (at high temperatures)
what removes the clathrin coat?
hsc70
-clathrin coat needs to be removed to fuse with endosomes
