Multidrug Resistance and Cystic Fibrosis Highlight a Family of Membrane Proteins with ATPBinding Cassette Domains

I. The Molecular Design of Life
13. Membrane Channels and Pumps
13.2. A Family of Membrane Proteins Uses ATP Hydrolysis to Pump Ions Across Membranes
Figure 13.7. Digitoxigenin. Cardiotonic steroids such as digitoxigenin inhibit the Na+-K+ pump by blocking the
dephosphorylation of E2-P.
I. The Molecular Design of Life
13. Membrane Channels and Pumps
13.2. A Family of Membrane Proteins Uses ATP Hydrolysis to Pump Ions Across Membranes
Foxglove (Digitalis purpurea) is the source of digitalis, one of the most widely used drugs. [Inga Spence/Visuals
Unlimited.]
I. The Molecular Design of Life
13. Membrane Channels and Pumps
13.3. Multidrug Resistance and Cystic Fibrosis Highlight a Family of Membrane
Proteins with ATP-Binding Cassette Domains
Tumor cells in culture often become resistant to drugs that were initially quite toxic to the cells. Remarkably, the
development of resistance to one drug also makes the cells less sensitive to a range of other compounds. This
phenomenon is known as multidrug resistance. In a significant discovery, the onset of multidrug resistance was found to
correlate with the expression and activity of a membrane protein with an apparent molecular mass of 170 kd. This
protein acts as an ATP-dependent pump that extrudes a wide range of small molecules from cells that express it. The
protein is called the multidrug resistance protein (MDR) or P-glycoprotein ("glyco" because it includes a carbohydrate
moiety). Thus, when cells are exposed to a drug, the MDR pumps the drug out of the cell before the drug can exert its
effects. A related protein was discovered through genetic studies of the hereditary disease cystic fibrosis (Section 1.1.4).
In one of the first studies leading to the identification of a specific genetic change causing human disease, investigators
performed a comparative genetic analysis of many people having this disease and family members who did not have the
disease. The gene found to be mutated in the affected persons encodes a protein, now called cystic fibrosis
transmembrane conductance regulator (CFTR). CFTR acts as an ATP-regulated chloride channel in the plasma
membranes of epithelial cells. As mentioned in Chapter 1, cystic fibrosis results from a decrease in fluid and salt
secretion by CFTR. As a consequence of this defect, secretion from the pancreas is blocked and heavy, dehydrated
mucus accumulates in the lungs, leading to chronic lung infections.
Analysis of the amino acid sequences of MDR, CFTR, and homo logous proteins revealed a common architecture
(Figure 13.8). Each protein comprises four domains: two membrane-binding domains of unknown structure and
two ATP-binding domains. The ATP-binding domains of these proteins are called ATP-binding cassettes (or ABCs) and
are homologous to domains in a large family of transport proteins of bacteria and archaea. Indeed, with 79 members, the
ABC proteins are the largest single family identified in the E. coli genome. The ABC proteins are members of the P-loop
NTPase superfamily (Section 9.4.4). Some ABC proteins, particularly those of prokaryotes, are multisubunit proteins
constructed such that the membrane-spanning domains and the ABC domains are present on separate polypeptide chains.
The consolidation of enzymatic activities of several polypeptide chains in prokaryotes to a single chain in eukaryotes is a
theme that we will see again (Section 22.4.4). For example, the histidine permease of Salmonella typhimurium, which
transports the amino acid histidine into the bacterium, consists of (1) two different protein subunits with membranespanning domains (HisQ and HisM) and (2) a homodimeric protein (HisP) with ABC domains (Figure 13.9). A soluble,
histidine-binding protein (HisJ) binds histidine after the amino acid enters the cell.
Like other members of the P-loop NTPase superfamily, proteins with ABC domains undergo conformational changes on
ATP binding and hydrolysis. These structural changes are coupled within each dimeric transporter unit in a manner that
allows these membrane proteins to drive the uptake or efflux of specific compounds or to act as gates for open
membrane channels.
I. The Molecular Design of Life
13. Membrane Channels and Pumps
13.3. Multidrug Resistance and Cystic Fibrosis Highlight a Family of Membrane Proteins with ATP-Binding Cassette Domains