Eukaryotic cells contain supportive proteins
that confer stability while allowing flexibility
(cytoskeletal proteins). The membrane skeleton
is a network of structural proteins underlying
the plasma membrane and partly associated
with it. Erythrocytes must meet extreme requirements:
about a half million times during a
4-month lifespan, they traverse small capillaries
with diameters less than that of the erythrocytes
themselves. Membrane flexibility is also
essential for muscle cell function. Thus, it is not
surprising that the cytoskeletal proteins of erythrocytes
and muscle cells are similar.
Sunday, April 12, 2009
Erythrocytes
A normal erythrocyte ismaintained in a characteristic
biconcave discoid form by the cytoskeletal
proteins. Genetic defects in different
cytoskeletal proteins lead to characteristic erythrocyte
deformations: as ellipses (elliptocytes),
as spheres (spherocytes), or as cells with
a mouthlike area (stomatocytes) or thornlike
projections (acanthocytes). The various forms
are the result of defects of different proteins.
biconcave discoid form by the cytoskeletal
proteins. Genetic defects in different
cytoskeletal proteins lead to characteristic erythrocyte
deformations: as ellipses (elliptocytes),
as spheres (spherocytes), or as cells with
a mouthlike area (stomatocytes) or thornlike
projections (acanthocytes). The various forms
are the result of defects of different proteins.
skeletal proteins in erythrocytes
SDS polyacrylamide gel electrophoresis differentiates
numerous membrane-associated erythrocyte
proteins. Each band of the gel is numbered,
and the individual proteins are assigned
to them. The main proteins include !- and "-
spectrin, ankyrin, an anion-channel protein
(band-3 protein), proteins 4.1 and 4.2, actin, and
others. The chromosomal localization of their
genes and associated diseases due to mutations
are known for man and mouse.
numerous membrane-associated erythrocyte
proteins. Each band of the gel is numbered,
and the individual proteins are assigned
to them. The main proteins include !- and "-
spectrin, ankyrin, an anion-channel protein
(band-3 protein), proteins 4.1 and 4.2, actin, and
others. The chromosomal localization of their
genes and associated diseases due to mutations
are known for man and mouse.
!- and "-Spectrin
The main component of cytoskeletal proteins is
spectrin, a long protein composed of a 260 kDa
! chain and a 225 kDa " chain. The chains consist
of 20 (! chain) and 18 (" chain) subunits,
each with 106 amino acids. Each subunit is
composed of three !-helical protein strands
running counter to one another. Subunit 10 and
subunit 20 of the ! chain consist of five, instead
of three, parallel chains. The individual subunits
are assigned to different domains
spectrin, a long protein composed of a 260 kDa
! chain and a 225 kDa " chain. The chains consist
of 20 (! chain) and 18 (" chain) subunits,
each with 106 amino acids. Each subunit is
composed of three !-helical protein strands
running counter to one another. Subunit 10 and
subunit 20 of the ! chain consist of five, instead
of three, parallel chains. The individual subunits
are assigned to different domains
Proteins of the erythrocyte membrane
The rod-shaped spectrin proteins, which run
parallel to the erythrocyte plasma membrane,
are attached to the anion channels by ankyrin
and to the glycophorin molecules by protein 4.1.
The anion channels in erythrocytes are important
for CO2 transport. Glycophorins (A, B, C) are
transmembrane proteins with several carbohydrate
units. Actin is themain protein for muscle
contraction and cell flexibility.
parallel to the erythrocyte plasma membrane,
are attached to the anion channels by ankyrin
and to the glycophorin molecules by protein 4.1.
The anion channels in erythrocytes are important
for CO2 transport. Glycophorins (A, B, C) are
transmembrane proteins with several carbohydrate
units. Actin is themain protein for muscle
contraction and cell flexibility.
Hereditary Muscle Diseases
Spontaneous degeneration of muscle fibers and
death of muscle cells (muscular dystrophy) is a
common cause of muscle disease in infants,
children, and adults. Muscular dystrophies are
genetically heterogeneous and clinically variable.
About 50 different forms are listed in
McKusicks’s catalogue Mendelian Inheritance
death of muscle cells (muscular dystrophy) is a
common cause of muscle disease in infants,
children, and adults. Muscular dystrophies are
genetically heterogeneous and clinically variable.
About 50 different forms are listed in
McKusicks’s catalogue Mendelian Inheritance
The dystrophin–glycan complex
A complex system of interconnected noncovalently
bound proteins in the sarcolemma
(plasma membrane) of muscle cells lends the
cell stability under the extreme exertion of contraction
and relaxation. They connect the extracellular
matrix and the intracellular myofibrils,
elongated protein molecules aligned in parallel
chains (myofilaments). The largest of the interconnected
proteins, !-dystroglycan (156 kDa),
is located outside the cell. It is connected to the
extracellular matrix by a heterotrimeric protein,
laminin-2. "-Dystroglycan (43 kDa) is embedded
in the sarcolemma and connected to a
series of other cytoskeletal proteins, which are
divided into the sarcoglycan and syntrophin
subcomplexes. Several members of the sarcoglycan
complex are related to specific types of
muscular dystrophies due to mutations in the
corresponding genes.
Dystrophin, a large, elongated protein, provides
a bridge between the intracellular cytoskeleton
involved in the contractile myofilaments and
the extracellular matrix. Two dystrophin
molecules connect neighboring dystrophin–
glycan complexes. The N-terminal end of dystrophin
is connected to the thinmyofilament Factin
(filamentous actin). The C-terminal end of
dystrophin is connected to "-dystroglycan and
the syntrophins.
bound proteins in the sarcolemma
(plasma membrane) of muscle cells lends the
cell stability under the extreme exertion of contraction
and relaxation. They connect the extracellular
matrix and the intracellular myofibrils,
elongated protein molecules aligned in parallel
chains (myofilaments). The largest of the interconnected
proteins, !-dystroglycan (156 kDa),
is located outside the cell. It is connected to the
extracellular matrix by a heterotrimeric protein,
laminin-2. "-Dystroglycan (43 kDa) is embedded
in the sarcolemma and connected to a
series of other cytoskeletal proteins, which are
divided into the sarcoglycan and syntrophin
subcomplexes. Several members of the sarcoglycan
complex are related to specific types of
muscular dystrophies due to mutations in the
corresponding genes.
Dystrophin, a large, elongated protein, provides
a bridge between the intracellular cytoskeleton
involved in the contractile myofilaments and
the extracellular matrix. Two dystrophin
molecules connect neighboring dystrophin–
glycan complexes. The N-terminal end of dystrophin
is connected to the thinmyofilament Factin
(filamentous actin). The C-terminal end of
dystrophin is connected to "-dystroglycan and
the syntrophins.
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