Receptors linked to G proteins - lipid and calcium signalling Flashcards
Most abundant lipid in biological membranes
Glycerophospholipids
Structure of glycerophospholipids
Two OH groups of glycerol attached via ester link to fatty acids
Third OH group linked to phosphate group (polar; negative charge)
Phosphate group linked to either choline; serine; inositol; or ethanolamine depending on type of phospholipid
Phosphoinositol lipids
Found in inner leaflet of membrane
Five phosphorylation sites on the inositol ring -> lots of information can be encoded in one small molecule
Functions of phosphoinositol lipids
1) as a source of diffusible signalling mediators
2) as a specific membrane-localised docking site that can recruit signalling molecules
How does IP3 regulate intracellular calcium levels?
IP3 diffuses through the cytosol to the endoplasmic reticulum (Ca2+ stores) where it binds to and activates IP3 receptors (IP3-gated Ca2+ channels).
IP3 diffuses through the cytosol to the endoplasmic reticulum (Ca2+ stores) where it binds to and activates IP3 receptors (IP3-gated Ca2+ channels).
IP3 inactivated by dephosphorylation
Protein kinase C (PKCs)
A family of serine/threonine kinases with at least 10 members
Structure of PKCs
N terminal regulatory domain and C terminal catalytic domain connected by a hinge region
Regulatory domain interacts with active site of catalytic domain & holds it in a low-activity conformation
Conventional activation of PKCs
In unstimulated cells, PKC in cytosol
When Ca2+ and DAG are present in sufficient concentrations (ie after activation of phospholipase C), they bind to the C2 and C1 domain, respectively, and recruit PKC to the membrane
Ca2+ binding to C2 domain causes membrane association
Diffuses in membrane until encounters DAG
DAG binding to C1 domain removes regulatory domain inhibition
Ca2+ as an intracellular signalling mediator
Cytosolic Ca2+ levels kept low (~10-7M) by Ca2+-ATPase pumps that use energy from ATP to pump Ca2+ out of the cell or into the intracellular Ca2+ stores (endoplasmic reticulum, sarcoplasmic reticulum in muscle cells)
Cytosolic Ca2+ also buffered by binding to proteins like calsequestin and by uptake into mitochondria
Cytosolic Ca2+ concentrations rise rapidly when Ca2+ channels open. These can be voltage-gated or ligand-gated channels in the plasma membrane that let in extracellular Ca2+, or IP3-gated channels on the intracellular Ca2+ store
Ca2+ signal tends to be very local and short-lived: Resting Ca2+ levels quickly restored by Ca2+-ATPase pumps; also high concentration of Ca2+ binding proteins in cytosol that bind Ca2+
The increase in cytosolic Ca2+ directly activates some effectors, and activates other via the Ca2+ binding protein calmodulin (CaM)
Examples of proteins regulated by Ca2+
Kinases
Ca2+ activated ion channels
Cytoskeletal proteins
Synaptic vesicle proteins
Calmodulin
Important 2+ sensor
Protein containing 4 EF-hand motifs
Conformation changes upon Ca2+ binding, exposing new peptide binding surface that interacts with and alters activity of target proteins
