Cell Phys Flashcards
What are ABC transporters?
- Transport system superfamily
- Largest protein family so far with 1-3% of genomes coding for their subunits
What is the general function of ABC transporters?
Unidirectional importing and exporting proteins against their chemical gradients
Eukaryotes- exporting
Describe the molecular structure of ABC transporters
Characterised by:
- Two nucleotide binding domains (NBDs)
- Two transmembrane domains (TMDs)
Also has:
- Phosphate- binding loop (P-loop)
- Short signature sequence (LSGGQ)
What have studies observed about the gating of ABC transporters?
Balarkrishnan et al. - drug efflux pump LmrA has been shown to be reversible in certain conditions.
This suggests that the membrane domain has ‘turnstile’ like gates rather than barriers.
What are the problems with the ATP switch model?
There is the general agreement that these steps must occur at some point but less understanding of the exact order.
List examples of human ABC transporters
P-glycoprotein (p-gp) MRP1 ABCA1 SUR1 (ABCC8) CFTR
Describe the functions of p-glycoprotein
- Transports neutral and cationic hydrophobic compounds
- Transports xenobiotics out of cells
- Exploited by tumour cells
- Expressed in filter organs (e.g. liver, kidney, intestine…)
What does p-gp have to do with multi-drug resistance?
Expressed at high levels in tumours -> transports anticancer drugs out of cancer cells
How does p-gp know what the cytotoxic level of anti-cancer drug is?
1- Already a small population of p-gp which survives and replicates via natural selection during cancer treatment
2- All cells have small concentration of p-gp but the instability of the cell cycle means that the tumour cell can up regulate p-gp expression
What is the role of MRP1?
Transports anionic compounds
What is MRP1 named a ‘multi-drug resistance associated protein’?
It’s expressed in most tissues (and therefore present in cancers).
This means that like p-gp, it can export anticancer drugs out of the cell.
What is the role of ABCA1?
Cholesterol efflux from:
- Macrophages
- Placenta
- Liver
- Lungs
- Adrenals
Describe the consequences of ABCA1 mutation
Tangier disease
Familial HDL deficiency, characterised by:
- Low HDL levels
- Lipid dense macrophage deposits in tissues
- Atherosclerosis and associated diseases
How does ABCA1 mutation cause familial HDL deficiency?
- Usually, app A can strip lipids from the membrane via the ABCA1 receptor
- ApoA can then get loaded, becoming a mature HDL particle which goes to the liver to get discarded.
- Stopping the ABCA1 receptor stops cholesterol and lipid from leaving the cell so it accumulated.
- Cells can get deposited anywhere - plaque formation more likely.
What is the role of SUR1?
- Interaction with KATP channels
- Insulin secretion
Describe the structure of the SUR1 receptor
- 4 SUR1 subunits
- Pore formed by 4 Kir6.2 subunits which potassium can move through when unblocked
How can the SUR1 channel be inhibited?
- Sulfonylureas (e.g. metformin) to NBD
- ATP or ADP to the Kir6.2 subunit
How can the SUR1 channel be stimulated?
MgATP and MgADP binding to NBD1 and NBD2
How do we know how KATP channels behave in insulin secretion?
- Calcium channels are always open whenever there is depolarisation
- Where calcium channels are open, potassium channels must be closed and vice versa
Describe medical problems which affect SUR1
Hyperinsulinism – beta islet cell membranes are always depolarised (due to MgADP insensitivity) -> increased insulin release-> low glucose
Neonatal diabetes – beta islet cell is always hyperpolarised (due to ATP insensitivity) so insulin is never released -> high glucose
What is the function of the ABCC7 transporter?
CTFR- transport of chloride ions out of cells
How is the gating of the CFTR protein regulated?
- Protein kinase A (PKA)- mediated phosphorylation
- ATP interaction (Muallem 2009)
- Nucleotide content of nucleotide binding domains (Wilkens 2015)
What is important for CFTR activation?
CFTR needs to be phosphorylated by PKA in the presence of ATP
What is the difference between SUR1 and CFTR?
SUR1 regulates electrical conductance, CFTR is just a chloride ion channel