64
chapter4
Three-Dimensional Structure of Proteins
of abnormal prion protein. The conversion of the normal
prion protein, whose function is unknown, to an aberrant
form involves a conformational change rather than a co-
valent modification. The abnormal prion protein functions
as a seed that induces the normal cellular prion protein to-
wards the abnormal amyloidogenic rich, /1-structure pro-
teins which can be propagated and transmitted to other
cells. The aggregated form of prion protein forming amy-
loid is resistant to proteolysis.
The
conversion
of
naturally
occurring
protease-
sensitive prion protein to a protease-resistanct form oc-
curs in vitro by mixing the two proteins. However, these
protease-resistant prion proteins are not infectious. Thus,
in the “protein-only” hypothesis of prion infection, the
acquisition of an abherrant conformation is not sufficient
for the propagation of infectivity. However, in the yeast
(Saccharomyces cerevisiae)
system, the abnormal prion
form of the yeast protein, introduced by liposome fusion,
is able to seed a self-propagating conformational change
of the normal proteins, which accumulate as aggregates.
The aggregates are transmissible to daughter yeast cells
along with the propagation of abnormal phenotype.
Recently a serious public health problem has arisen by
showing that a prion disease in cattle can cross species bar-
riers and infect humans. This occurred when cattle were
fed meal made from sheep infected with scrapie. The cat-
tle developed BSE (commonly called “mad cow disease”).
Subsequently, when people consumed prion-contaminated
beef, a small number, primarily in Great Britain, devel-
oped a variant of CJD (vCJD) approximately five years
afterward. The variant form of CJD is a unique form of
prion disease occurring in a much younger population than
would be expected from inherited or sporadic CJD. Both
BSE and vCJD share many similar pathologic character-
istics suggesting an etiologic link between human vCJD
and cattle BSE.
The tumor suppressor protein
p53
provides yet another
example of protein misfolding that can lead to pathological
effects, in this case cancers
(p
is for protein and 53 is for
its approximate molecular weight of 53,000). The gene for
p53
is located on the short arm of chromosome 17 (17/?)
and codes for a 393-amino-acid phosphoprotein. In many
cancers the
p53
gene is mutated and the lack of normal
p53
protein has been linked to the development of as many
as 40% of human cancers.
Normal
p53
functions as a tumor suppressor and is a
transcription factor that normally participates in the reg-
ulation of several genes required to control
cell growth,
DNA repair,
and
apoptosis
(programmed cell death). Nor-
mal
p53
is a tetramer and it binds to DNA in a sequence-
specific manner. One of the p53-regulated genes produces
a protein known as
p21
, which interferes with the cell cycle
by binding to cyclin kinases. Other genes regulated by
p53
are MDM2 and BAX. The former gene codes for a protein
that inhibits the action of
p53
by functioning as a part of
a regulatory feedback mechanism. The protein made by
the BAX gene is thought to play a role in p53-induced
apoptosis.
Most mutations of
p53
genes are somatic missense
mutations involving amino acid substitutions in the DNA
binding domain. The mutant forms of
p53
are misfolded
proteins with abnormal conformations and the inability
to bind to DNA, or they are less stable. Individuals with
the rare disorder
Li-Fraumeni syndrome,
(an autosomal
dominant trait) have one mutated
p53
gene and one normal
p53
gene. These individuals have increased susceptibility
to many cancers, such as leukemia, breast carcinomas,
soft-tissue sarcomas, brain tumors, and osteosarcomas.
Clinical trials are underway to investigate whether the
introduction of normal
p53
gene into tumor cells by means
of gene therapy (Chapter 23) has beneficial effects in the
treatment of cancer. Early results with
p53
gene ther-
apy indicate that it may shrink the tumor by triggering
apoptosis.
Supplemental Readings and References
Protein Folding and Its Defects
R. Aurora, T. R Creamer, R. Srinivasan and G. D. Rose: Local interactions in
protein folding: Lessons from thear-helix.
Journal of Biological Chemistry
1 1 1 ,
1413(1997).
J. R. Beasley and M. H. Hecht: Protein design: The choice of de novo
sequences.
Journal of Biological Chemistry
1 1 1 ,
2031 (1997).
M. Blaber, X.-J. Zhang, and B. W. Mathews: Structural basis of amino acid
a-helix propensity.
Science
260, 1637 (1993).
R. W. Carrell and D. A. Lomas: Conformational disease.
Lancet
350, 134
(1997).
W. D. Kohn, C. T. Mant, and R. S. Hodges: a-helical protein assembly
motifs.
Journal of Biological Chemistry
1 1 1 ,
2583 (1997).
R. W. Ruddon and E. Bedows: Assisted protein folding.
Journal of Biological
Chemistry
1 1 1 ,
3125 (1997).
P. J. Thomas, B.-H. Qu, and P. L. Pedersen: Defective protein folding as a
basis of human disease.
Trends in Biochemical Sciences
20, 456 (1995).
Alzheimer’s Disease,
p5
3,
and Prions
J. Avila: Tau aggregation into fibrillar polymers: taupathies.
FEBS Letters
476,
89 (2000).
A. Bossers, R. de Vries, M. A. Smits: Susceptibility of sheep for scrapie as
assessed by in vitro conversion of nine naturally occurring variants of PrP.
Journal of Virology
74,
1407 (2000).
F. E. Cohen: Prion, peptides and protein misfolding.
Molecular Medicine
Today
6,292 (2000).
