integrins lect 29-30 Flashcards
what are intergins?
major metazoan receptor for cell adhesion
integrins are receptors for what organisms?
viruses and bacteria
what is the structure of integrins?
heterodimers; combination of 1 alpha and 1 beta subunit
how many types of integrins exist?
24 different combinasons
what subunit is synthesized in excess – pairs with many α-subunits?
B1
what are the 4 subfamilies of integrins?
- laminin receptors
- RDG receptors
- collagen receptors
- leukocyte-specific receptors
which integrin subunit has a di-sulfide bond?
alpha
beta subunit has a ________-rich domain
cysteines
many alpha subunit undergo what type of post-translational modification?
endoproteolytic processing at pairs of basic amino acids that is mediated by furin
what structure is found in all alpha subunits?
7 beta propeller motif
where is the MIDAS motif found in integrins?
- alpha subunit: in the 7 B propeller motif, in the I/A domain
- beta: toward N term
which integrin is the only one to not bind actin cytoskeleton? what does it bind?
a6B4: binds intermediate filaments
do all alpha subunits have a I/A motif in alpha subunit containing MIDAS?
no
what amino acids are almost always found in integrin-binding sequences?
D (aspartic acid) or E (glutamic acid)
what is the aVb3 integrin?
- no I/A domain (therefore no MIDAS domain) in alpha subunit
- involved in angiogenesis
describe an example of an integrin ligand that has a synergy site
fibronectin:
- FN9 = synergy site binding alpha subunit
- FN10 = RDG site binding beta subunit
what integrin does fibronectin bind to?
a5B1
how are fibronectin fibers formed?
fibronectin binds to integrin which are bound to the actin cytoskeleton.
fibronection dimerize and move following the actin/myosin interactions
name every player involved in fibronectin-actin linking
fibronection, integrin a5B1, actin binding proteins (vinculin, talin, paxillin, FAK), actin
what is cytochalasin?
drug that inhibit actin polimeryzation.
it also dissociates fibronectin because it is thightly linked to actin
what do disintegrins do?
proteins that block the function of integrins
what are 2 effects of disintegrins and via what integrins do they act?
- block coagulation via aIIbB3
- block angiogenesis via
aVB3
where are disintegrins found?
in snake venom
what is in their sequence?
cysteine rich and RGD motif
name 3 disintegrins
Eristostatin, Echistatin, Kistrin
disrupting the different beta / alpha genes gave what results?
different phenotypes for each! shwoing their very different functions
what disease is caused by a6B4 KO?
epidermis bullosa (lots of blistering)
what disease is caused by aIIbB3 KO?
glanzmann thrombostheria (bleeding; like disintegrin effect)
what disease is caused by B2 subunit KO?
inhibits leucocyte migration / adhesion to inflammation site
more specifcially what type of epidermis bullosa is caused by a6B4 mutation?
hemidesmosal (disruption of hemidesmosome attachment of keratinocytes hemidesmosomes to BM laminin)
what other mutations can cause epidermis bullosa? what type?
- keratin mutation = simplex
- plectin, a6B4 = hemodesmosal
- laminin = junctional
- collagen = dystrophic
what are the 2 ways how integrins can get activated?
- outside via ligand binding
- inside via cytoplasmic tail separation
(some are constituvely active)
what movement of the subunit must happen for integrins to be activated?
separation of the alpha and beta subunit
name one activator that helps separate integrin cytoplasmic tails?
talin
what integrin is mostly in an inactive state (activation step is important for it)
aIIbB3
how do integrins that do not have an alpha I/A domain get acti vated by a ligand? name an example
a4B6:
1- RDG domains of fibronectin binds beta I/A
2- C term of I/A moves down
3- integrin opens up
4- synergic site of fibronectin can bind alpha subunit
how do integrins that have an alpha I/A domain like a2B1 get activated by a ligand?
1) collagen binds alpha I/A domain
2) C term helix moves down
3) makes beta I/A transmit conformational change to hybrid domain
what links the cytoplasmic tails of the integrin subunits in an inactive state?
salt bring: ionic non-covalent interaction
what is talin’s effect on integrins?
bind beta subunit to disrupt the salt bridge, leading to integrin activation
how can talin get activated?
- cleavage relesing its integrin binding head
- PIP2 binding
what regulates integrin activation?
talin and kindlin
what does kindlin do?
connect integrin B subunit to actin via other adaptor proteins
what is ILK?
integrin linked (pseudo)kinase: binds B subunit to actin via actin binding proteins and PINCH
where do integrins cluster?
focal adhesion points!
what is paxillin?
actin binding protein that binds ILK and actin
why is integrin clustering important?
integrins alone have low affinity for their ligand
remember: integrins bind actin via what end?
intracellular cytoplasmic tail
integrins have low affinity but high what?
avidity (combined strength of multiple bond interactions)
how does the integrin aIIbB3 get activated?
outside-in atcivation via binding to collagen outside blood vessels or thrombin
once activated, what does aIIbB3 bind?
fibrinogen to aggregate platelets
what are the steps of leucocyte extravasion?
1) weak interaction with selectins on endothelial cells
2) chemokine release -> GPCR -> integrin (ax a4B1) clustering
3) integrins can now interact with ICAMs
4) leucocytes enters tissue
what integrin is involved in leucocyte extravasion?
a4B1
check last slide integrins involvement in cancer / angiogenesis
