162 Chapter 4 T H E S T R U C T U R E A N D F U N C T I O N O F T H E P L A S M A M E M B R A N E
Several severe, inherited disorders have been traced to mutations
in genes that encode ion channel proteins (Table 1).
Most of the disorders listed in Table 1 affect the movement
of ions across the plasma membranes of excitable cells (i.e.,
muscle, nerve, and sensory cells), reducing the ability of these
cells to develop or transmit impulses (action potentials, page
167). In contrast, cystic fibrosis, the best-studied and most
common inherited ion channel disorder, results from a defect
in the ion channels of epithelial cells.
On average, 1 out of every 25 persons of Northern European
descent carries one copy of the mutant gene that can
cause cystic fibrosis. Because they show no symptoms of the
mutant gene, most heterozygotes are unaware that they are
carriers. Consequently, approximately 1 out of every 2500 infants
in this Caucasian population (1/25 1/251/4) is homozygous
recessive at this locus and born with cystic fibrosis
(CF). Although cystic fibrosis affects various organs, including
the intestine, pancreas, sweat glands, and reproductive
tract, the respiratory tract usually exhibits the most severe effects.
Victims of CF produce a thickened, sticky mucus that is
very hard to propel out of the airways. Afflicted individuals
typically suffer from chronic lung infections and inflammation,
which progressively destroy pulmonary function.
The gene responsible for cystic fibrosis was isolated in
1989. Once the sequence of the CF gene was determined and
the amino acid sequence of the corresponding polypeptide was
deduced, it was apparent that the polypeptide was a member of
the ABC transporter superfamily. The protein was named cystic
fibrosis transmembrane conductance regulator (CFTR), an ambiguous
term that reflected the fact that researchers weren’t sure
of its precise function. The question was answered after the
protein was purified, incorporated into artificial lipid bilayers,
and shown to act as a cyclic AMP-regulated chloride channel,
not a transporter. Subsequent studies revealed that CFTR also
functions in the conductance of bicarbonate ( ) ions.
The cells of patients with cystic fibrosis exhibit an abnormally
low efflux of chloride and/or bicarbonate ions, with
the particular defect depending upon the specific site of mutation
within the protein. It remains uncertain, however,
how this underlying molecular defect leads to the development
of chronic lung infections. Numerous explanations
have been offered, including the following:
1. Because the movement of water out of epithelial cells by
osmosis follows the movement of salts, the decrease in Cl
efflux caused by CFTR deficiency could lead to a decrease in
the volume of fluid that bathes the epithelial cells of the
airways. A reduction in volume of the surface liquid, and a
resulting increase in viscosity of the secreted mucus, could
impair the function of the cilia that are responsible for
pushing bacteria out of the respiratory tract.
2. Among the bacteria that infect the airways of CF patients,
Pseudomonas aeruginosa is the most prevalent and destructive
species (Figure 1). This bacterium is rarely seen in the airways
of individuals suffering from other types of lung disease, and
HCO
3
THE HUMAN PERSPECTIVE
Defects in Ion Channels as a Cause
of Inherited Disease
Inherited disorder Type of channel Gene Clinical consequences
Familial hemiplegic migraine (FHM) Ca2 CACNL1A4 Migraine headaches
Episodic ataxia type-2 (EA-2) Ca2 CACNL1A4 Ataxia (lack of balance and coordination)
Hypokalemic periodic paralysis Ca2 CACNL1A3 Periodic myotonia (muscle stiffness) and paralysis
Episodic ataxia type-1 K KCNA1 Ataxia
Benign familial neonatal convulsions K KCNQ2 Epileptic convulsions
Nonsyndromic dominant deafness K KCNQ4 Deafness
Long QT syndrome K HERG Dizziness, sudden death from ventricular
KCNQ1, or fibrillation
Na SCN5A
Hyperkalemic periodic paralysis Na SCN4A Periodic myotonia and paralysis
Liddle Syndrome Na -ENaC Hypertension (high blood pressure)
Myasthenia gravis Na nAChR Muscle weakness
Dent’s disease Cl CLCN5 Kidney stones
Myotonia congenita Cl CLC-1 Periodic myotonia
Bartter’s syndrome type IV Cl CLC-Kb Kidney dysfunction, deafness
Cystic fibrosis Cl CFTR Lung congestion and infections
Cardiac arrhythmias Na many different Irregular or rapid heartbeat
K genes
Ca2
Table 1
4.7 T H E M O V E M E N T O F S U B S T A N C E S A C R O S S C E L L M E M B R A N E S 163
it is not certain why CF patients are so susceptible to it.
Studies indicate that P. aeruginosa binds to the extracellular
end of the CFTR protein, and it is speculated that this
attachment may lead to ingestion and destruction of the
bacterium by the epithelial cells. Individuals who lack the
CFTR protein in their plasma membrane, as do many CF
patients (see the following), may be unable to clear the
bacterium from their respiratory tract. It is also speculated
that decreased bicarbonate ion secretion could lower the pH
of the fluid that lines the respiratory airway and provide a
more hospitable environment for bacterial growth.
In the past decade, researchers have identified more than
800 different mutations that give rise to cystic fibrosis, and the
effect of many of these alterations on protein structure has been
studied (Figure 2). Nearly 70 percent of the alleles responsible
for cystic fibrosis in the United States contain the same genetic
alteration (designated F508)—they are all missing three base
pairs of DNA that encode a phenylalanine at position 508,
within one of the nucleotide-binding domains of the CFTR
polypeptide. Subsequent research has revealed that CFTR
polypeptides lacking this particular amino acid fail to be
processed normally within the membranes of the endoplasmic
reticulum and, in fact, never reach the surface of epithelial cells.
As a result, CF patients who are homozygous for the F508 allele
completely lack the CFTR chloride channel in their plasma
membranes and have a severe form of the disease. Other CF
patients with less severe forms have mutant alleles that encode a
CFTR that is able to reach the surface of cells but mediates a reduced
chloride conductance. The mildest forms are characterized
by infertility, with little or no damage to major organs.
According to one estimate, the F508 mutation had to
have originated more than 50,000 years ago to have reached
such a high frequency in the population. The fact that the CF
gene has reached this frequency suggests that heterozygotes
may receive some selective advantage over those lacking a
copy of the defective gene. It has been proposed that CF heterozygotes
may be protected from the effects of cholera, a
disease that is characterized by excessive fluid secretion by
the wall of the intestine. One difficulty with this proposal is
that there is no record of cholera epidemics in Europe until
the 1820s. An alternate proposal suggests that heterozygotes
are protected from typhoid fever because the bacterium responsible
for this disease adheres poorly to the wall of an intestine
having a reduced number of CFTR molecules.
Ever since the isolation of the gene responsible for CF, the
development of a cure by gene therapy—that is, by replacement
BACTERIUM CILIA
FIGURE 1 The rod-shaped bacterium P. aeruginosa growing on cultured
epithelial cells from the human respiratory tract. (COURTESY
OF THOMAS MONINGER, REPRINTED WITH PERMISSION FROM NATURE
406:948, 2000; COPYRIGHT 2000, MACMILLANMAGAZINES LIMITED.)
1
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3 4 5 6a 6b 7 8 9 10 11 12
NBD 1 NBD 2
R Domain
13 14a14b 15 1617a 17b 18 19 20 21 22 23 24
FIGURE 2 Identifying the mutations responsible for cystic fibrosis.
The location and type of more than 150 different CF mutations
that have been identified by the Cystic Fibrosis Genetic
Analysis Consortium are indicated below a schematic of the
CFTR protein. () In-frame deletion; (■) missense mutation;
() nonsense mutation; () frame-shift mutation; and ()
splicing mutation. Most of these classes of mutation are discussed
in Chapter 11. This figure indicates the variety of alleles
of a gene that can exist within a population. The numbers correspond
to the coding segments (exons) that make up the gene.
The F508 deletion common in persons of Northern European
descent occurs in the 10th exon. (REPRINTED WITH PERMISSION
FROM F. S. COLLINS, SCIENCE 256:775, 1992; COPYRIGHT 1992,
AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE.)
Using Light Energy to Actively Transport Ions Halobacterium
salinarium (previously H. halobium) is an archaebacterium
that lives in extremely salty environments, such as
that found in the Great Salt Lake. When grown under
anaerobic conditions, the plasma membranes of these
prokaryotes take on a purple color due to the presence of
one particular protein, bacteriorhodopsin. As shown in Figure
4.47, bacteriorhodopsin contains retinal, the same prosthetic
group present in rhodopsin, the light-absorbing
protein of the rods of the vertebrate retina. The absorption
of light energy by the retinal group induces a series of conformational
changes in the protein that cause a proton to
move from the retinal group, through a channel in the protein,
to the cell exterior (Figure 4.47). The proton donated
by the photo-excited retinal is replaced by another proton
transferred to the protein from the cytoplasm. In effect, this
process results in the translocation of protons from the cytoplasm
to the external environment, thereby generating a
ste