Never Work With Membrane Proteins Quiz

Answer: The voltage across the membrane

Ions are charged atoms or molecules. The plasma membrane is analogous to a capacitor, storing and separating charge. The voltage across the membrane varies from cell to cell, but a typical value is that the inside of the cell is -65 mV (millivolts) relative to the outside.

This may suggest that positive ions will flow into the cell when ion channels open, but, as mentioned in the question, the movement of ions is determined by both concentration and voltage. This so called "electrochemical potential" can be calculated using the Nernst equation.

Answer: Aquaporins

Although water molecules are small enough to pass through the plasma membrane without the help of proteins, the rate of this flow is very slow and is unlikely to protect cells from bursting if they were placed into a hypotonic solution (i.e. a solution with lots of water relative to solute).

These proteins are not gated, like the ion channels discussed in question 1, but are constitutively open. Aquaporins are able to selectively allow the passage of water molecules whilst rejecting other substances, even those smaller than water.

This is done by utilising a selectivity filter in the centre of the pore, which rejects the entry of hydrogen ions, that would otherwise result in the acidification of the cell.

Answer: The Extracellular Matrix

The extracellular matrix (ECM) is an often overlooked component of our bodies. It surrounds just about every cell in our body and is essential to life. It contains several structural proteins, to which proteins such as integrins can attach and anchor the cell. Proteins in the ECM also sequester important growth factors which can be released in times of stress, promoting cell survival.

Integrins play an important structural role (e.g. cell anchorage and cell motility), but are also important in signalling. By establishing contacts with the ECM, integrins tell the cell that they are in a stable environment and so encourage its survival.

Answer: Cadherins

Cadherins are involved in calcium-dependent homotypic interactions. This means that a cadherin from one cell binds to a similar cadherin from a neighbouring cell, and that this interaction requires calcium ions. As mentioned in the question, cadherin contacts are often lost when cancerous cells metastasise. Understanding the biology of cadherins is therefore important in developing drugs that can prevent metastasis.

Answer: ATP

Pump proteins often consist of more than one subunit and their conformational changes have been characterised using X-ray crystallography - a technique that provides high resolution images of proteins. These conformational changes assist the movement of ions (and other species) across the cell membrane against their electrochemical gradient, and the changes are coupled to ATP binding, as well as ATP hydrolysis.

Answer: It prevents drugs from doing their jobs in cells

Pgp is particularly highly expressed in brain and so is a major hurdle to be overcome for drugs which target brain tumours. Like other ABC transporters, Pgp consists of two membrane spanning domains and two nucleotide-binding domains. Unlike the pump proteins, ATP hydrolysis (which is what releases this molecule's energy) is not required to transport the substance across the membrane, but is instead needed to reset the ABC transporter's original conformation.

Answer: Secondary transporters

Unlike ABC transporters and pumps, the energy utilised by secondary transporters is not from ATP, but the stored energy of pre-existing concentration gradients generated by pump proteins. The energy released when this gradient is reduced is used to transport another ion species against its concentration gradient. Secondary transporters almost always involve the movement of sodium and/or hydrogen ions.

Answer: Second messengers

Second messengers are released and can diffuse through the cell to target regions where they bring about the next response in the signalling cascade. For example, cAMP, released by adenylyl cyclase, is released and binds and activates protein kinase A, which then phosphorylates several targets. Calcium and certain lipids can also be described as second messengers.

Answer: Receptor Tyrosine Kinases

The receptor tyrosine kinase (RTK) family is large and is involved in several key processes. Insulin binds to the insulin receptor and promotes glucose metabolism and cell proliferation. Vascular endothelial growth factor (VEGF) binds its receptor (also an RTK) and promotes angiogenesis (the growth of blood vessels).

The latter example is particularly important in cancer development. Cancer cells often have overactive VEGF signalling, leading to increased vascularisation and improved oxygen supply to tumours.

Answer: Rhodopsin - which helps us see

Rhodopsin, along with its bacterial analogue bacteriorhodopsin, was the first GPCR to be discovered, though it was not classified as a GPCR until much later. Studies on another protein known as the beta adrenergic receptor - which explains our responsiveness to adrenaline - showed it to be structurally similar to rhodopsin. Specifically, these proteins are characterised by seven membrane-spanning domains.

The study of beta adrenergic receptors (and GPCRs in general) continued, looking at its structure, as well as its role in signalling.

This led to the 2012 Nobel Prize being awarded to Robert J. Lefkowitz and Brian K. Kobilka for their work with GPCRs. The difficulty in studying the crystal structures of GPCRs has meant that many of their complexities remain to be elucidated. Coupled with the fact that so many functions are regulated by GPCRs, the continued study of these proteins should be encouraged.