Ages ago I wrote about how the molecules that cells need (or need to get rid of) travel through the membrane. Now it’s time to consider the problem of transmission of information through the barrier.
First of all we should consider the type of information that a cell receives.
- Hormonal signals e.g. insulin. This is released from the pancreas when blood sugar levels are high. It mainly acts on liver and muscle cells causing a increase of uptake of glucose and subsequent storage as glycogen within the cells.
- Neurotransmitters e.g. acetylcholine. This is released from nerve cells and makes skeletal muscles contract. It also released into the synapase or between two nerve cell, transmitting the signal from one to the other.
- “Environmental conditions” e.g. glucose. Bacteria are able to swim in the direction of increasing glucose concentration. The process of moving in a certain direction based on chemical signals is known as chemotaxis. Also, we can smell all sorts of different chemicals.
There are other classes too but these are the main types that I want to consider today as they illustrate the different mechanisms across membranes but also reinforce the fact that mechanisms are shared across a wide range of different functions.
A key, take home point is that every molecular signal has a receptor. This is a protein that binds the signalling molecule and then passes the message on to the rest of the cell.
Some signalling molecules are able to move through the cell membrane independently. These are generally lipophilic (lipid loving) hormones such as testosterone and cortisol. Once inside the cell, an intracellular receptor binds the hormone and a message is passed down a chain of more proteins until an effector protein is activated causing a response. This doesn’t really involve the membrane so this is as much as I’m going to say about this!
Most hormones and “environmental signals”, and all neurotransmitters bind their receptors at the cell membrane – the receptors are membrane proteins.
One way of getting a message across is through dimerisation, the coming together of two proteins which serves to bridge the inside and outside of the cell. Binding of the signal outside causes two proteins to interact and transfer chemical groups. This changes the structure and allows the binding of further “downstream” proteins and the signal is passed down a chain until the effector is reached. Insulin receptors are believed to act like this, leading to effects such as increased production of glucose transporters.
Another mechanism for signal across a membrane is to activate a enzyme that catalyses the production of a so called “second messenger”. One very large family of proteins that do this are called G-protein-coupled receptors (GPRCs). These bind the signalling molecule, change shape and activates G-protein allowing it to interact with other membrane proteins including adenylate cylase (which makes a chemical known as cAMP), phospholipases (which break apart membrane phospholipids) and ion channels (which allow the flow of ions into the cell). These secondary chemical then diffuses through the cell where it binds other proteins to cause effects. Smell sensors or olfactory receptors are GPRCs!
The final mechanism I’ll talk about today are ligand-gated ion channels (LGICs). These channels open in response to the direct binding of a chemical to the channel such as a neurotransmitter. Binding of acetylcholine to receptors on a muscle cell opens the channel and allows the exit of potassium ions but a much greater entry of sodium ions. This changes the electrical properties of the membrane opening voltage gated calcium channels, causing the muscle fibres to contract. Magic, and not possible without membrane proteins!
Marshallian Surplus 