Lectures 7-9: Plasma Membrane Proteins

Oxidative stress & chaperones

Oxidative stress targets already unfolded proteins

More efficient to repair misfolded proteins than translating new ones

Chaperones repair proteins

Chaperone function

Surrounds & protects proteins while folding to avoid factors that affect protein conformation

Gap junctions

Allows for direct communication between cells

Channels allow ions, metabolites, signalling molecules (cAMP) & electric impulses to pass through

In nerves, heart, endothelium & smooth muscle

Gap junction structure

One hemichannel from each plasma membrane

6 connexins form 1 connexon (hemichannel)

1 connexon from each cell form a gap junction

Astrocyte functions

Induces vasodilation & constriction via brain capillaries to shift blood flow from one region of the brain to another

Converts glucose to lactate for energy for neurons

Secretes nerve growth factor for nervous tissue development

Communicates electrically w/ neurons to modulate synaptic communication

Reuptakes NTs to stop signalling

Astrocytic syncytium

Connected by gap junctions

Ca2+ waves travel through the syncytium induced by mechanical stimulation & glutamate

Controls extracellular concentration of molecules → Ca2+ influx causes release or uptake of ions & neuromodulators

Protects extracellular/other cells from excess glutamate (toxic) & K+

Astrocytic syncytium in disease

Epilepsy, Alzheimer’s & stroke have significant changes in astrocytic [Ca2+] & in duration/amplitude of Ca2+ waves

Abnormal connexon function after ischemic insults & traumatic brain injuries 

Integrins

Collagen - structure & support in ECM

Fibronectin - glycoprotein binds collagen & integrins (bridge)

Actin - cytoskeleton & provides shape, support, movement

Integrin dimer - binds proteins in & out of cell

Essential for anchorage/attachment to plasma membrane

Signalling pathways & bio processes (communication)

Extracellular part binds molecules in ECM or surface of other cells

Attachments to other cells can break & reform w/ cell movement

Voltage-gated ion channel mechanism

Helices w/ charged amino acids face one side (closed)

Membrane depolarisation causes helix to rotate opening a channel pore to allow ion flow

N-type inactivation (ball & chain)

Depolarisation opens a voltage gated ion channel

Ball & chain structure plugs the pore to stop ion flow (still depol.)

Prevents continuous ion flow & ensures brief & direct signals

Membrane can repolarise & close 

Ball & chain displaced when ion channel is closed for the next signal

GPCR stucture

7 transmembrane spanes

N-terminus = extracellular

C-terminus = cytoplasmic

Signal binding site & segment that interacts w/ G proteins

G protein structure

Heterotrimeric

15 α, 6 β, & 12 γ subunits

G_a activation triggers GTP binding → dissociates into G_a & Gβγ dimer (attached to receptor)

Phototransduction mechanism

cGMP is produced by guanylyl cyclase in the dark

cGMP binds & activates Na+ channels 

Channel opens & Na+ enters & depolarises membrane (not an action potential)

Depolarisation triggers glutamate release to send signal

μOpioid receptor signalling

GPCR that activates 2 pathways w/ morphine & SR-11501

G protein pathway causes painkilling effect (analgesic)

β-arrestin-2 pathway contributes to respiratory depression & constipation

Morphine & other opioids activate both pathways

Risk of respiratory failure from taking them

Biased agonist opioid receptor signalling

SR-17018

Preferentially activates G protein signalling for painkilling but not β-arrestin-2 signalling 

No respiratory depression 

Non-canonical GPCR signalling

Doesn’t follow typical GPCR signalling or usual G proteins

Signals via interactions w/ β-arrestins, direct receptor-kinase interactions &/or altering intracellular conformation

GPR50 = orphan receptor w/ a carboxyl end that when cleaved by Ca2+-Calpain translocates to nucleus & activates TFs directly

Sodium-glucose transporters (SGLTs)

Specialised carrier proteins that facilitate sodium-glucose co-transport w/o direct ATP use (intestinal epithelial cells)

Na+/K+ ATPase pumps Na+ out of the cell & lowers the intracellular concentration

This creates an Na+ gradient that drives glucose uptake into the cell along w/ more Na+ (symport)

Secondary active transport

SGLT1

Small intestine & late proximal tubule of kidney

2 Na+ & 1 glucose 

High affinity → efficient glucose absorption at low concentrations

SGLT2

Early segments of proximal kidney tubule

Lower affinity for glucose but handles 90% of glucose reabsorption in the kidney

Na+/K+ ATPase pump

3 Na+ out, 2 K+ in

Maintains low intracellular Na+ & high intracellular K+

Autophosphorylates after ATP hydrolysis

Phosphorylation changes conformation to expose ion binding site on extracellular & cytosolic faces

1/3 of cell’s energy fuels Na+/K+ pump

Pinocytosis

Internalisation of fluids into a cell via membrane vesicle