Structure and gating mechanism of the acetylcholine receptor pore

Acetylcholine receptor Important in transmitting messages from neuron to neuron Is a excitatory neurotransmitter Acetylcholine is stored in vesicles at the end of a neuron When the right signal comes along, it causes the release of acetylcholine into the synaptic cleft Here it binds to the acetylcholine receptor and which will allow for further transmission of the signal

Propagation of signal from neuron to neuron via neurotransmitters

Acetylcholine receptor Classic example of a ligand gated channel So it is a ion channel that is ligand gated It ultimately leads to an influx of positive ions

Ach Receptor Structure 5 transmembrane polypeptides and they are 2 alpha, 1 beta, 1 delta, and 1 gamma Each subunit is made up of 4 transmembrane alpha helices (helical bundle). They are M1-M4

Picture of subunits and M1-M4

Each helices has features M2 is mostly amphipathic while M1, M3, and M4 are mostly hydrophobic M2 has polar side chains for the most part in the middle of the pore and hydrophobic backing up against the back of the pore

Inside the channel In the middle of the channel there are 5 leucine sidechains and these 5 leucine sidechains come 1 from each M2 helix There function is to protrude into the channel and block it This is when the channel is in its closed conformation and inactivated

Picture inside the channel

The activated Ach Receptor Bulky hydrophobic leucines are removed from channel core This occurs when Ach binds to the receptor Binding causes conformational change, twisting of the M2 helices and the leucines twist out of the way

The Activated Ach Receptor continued The M2 helices now instead of having bulky hydrophobic residues now have polar residues facing inward and ions can go down the channel

Picture of Activated Ach Receptor

Ach Receptor Summary Bulky hydrophobic leucine side chains of M2 helices close the channel Upon activation from Ach binding, the leucine side chains twist out of the way and now the polar hydrophilic side chains and now positive ions can rush through.

The Paper about Ach Receptor Used liquid high helium temperatures Resolved protein structure at 4A Proposed a mechanism of how the pore opens and closes in response to presence or absence of Ach

Method Obtained crystals from Torpedo marmorata membranes Crystals at first did not yield high amplitudes so they adjusted their procedure using Fourier transforms to enhance signal Used electron imaging to obtain their structures

Signal Transduction When cells convert an extracellular signal to an intracellular signal that alter the behavior of the target cell

Signal Transduction

Recptors Ionotropic Channel Very fast Metabotropic Second messenger slower

Receptors Ion-channel-linked (AchR) ionotropic G-protein linked metabotropic Enzyme-linked metabotropic involves the covalent modification of P to serine threonine or tyrosine

G protein receptor 7 transmembrane helices Links outside to inside of cell

G protein Made up of three subunits, alpha, beta, and gamma ( A heterotrimeric protein) The alpha subunit is referred to as the key player in relaying the signal

Mechanism of G protein

Activated G protein leads to the exchange of GDP (inactive form) to GTP (active form) Then this activated subunit will then go and activate another protein

Alpha Subunit GTP will bind at a site on this subunit and activate it G  from Transducin

Beta Subunit Built from 7 WD repeats ( motifs that are built from tryptophan(W) and aspartate(D) Beta Propellar formed by seven subunits made of Beta Sheets Pic of Beta Subunit

Putting it all together G  does not interact with G   but the are linked by the G   subunit G  G   interact Forms functional signaling unit

GDPGTP GEF GAP

Ras protein Monomeric Is active when it has GTP bound Play a role in cell growth and differentiation GTP linked to Ras via Mg 2+

Ras protein

Ras protein with GTP linked to it via Mg2+