absence of ATP. In skeletal muscle, phosphorylation of the light chain is not
required for binding to actin (answer e).
129.The answer is c.(Alberts, p 1299. Kasper, pp 2526–2529. Kumar, pp
1336–1338.)Dystrophin, like α-actinin and spectrin, is an actin-binding
protein. It binds actin to the skeletal muscle membrane and therefore binds
the I and M bands to the cell membrane. The inability to bind actin to the
plasma membrane of skeletal muscle leads to disruption of the contraction
process, weakness of muscle, and abnormal running, hopping, and jump-
ing. The Gower maneuver is the method used by persons suffering from
muscular dystrophy to stand from a sitting position. Respiratory failure
occurs in those persons because of disruption of diaphragmatic function. Dys-
trophin is found in all muscle types and is part of a complex that regulates
interactions of the sarcolemma with the extracellular matrix through associ-
ated glycoproteins (dystrophin-glycoprotein complex). Therefore, loss of
dystrophin causes destabilization and decreased contractility (answer a).
Muscular dystrophy refers to a group of progressive hereditary disorders
(1/3500 male births) that involve mutations in the dystrophin gene. Dys-
trophin is similar in structure to spectrins I and II and α-actinin. Dystrophin
is absent in Duchenne muscular dystrophy. Becker muscular dystrophy is a
less severe dystrophy in which dystrophin is defective. Synthesis of actin
(answer b)is not reduced in skeletal muscle from these patients; in fact,
hypertrophy and pseudohypertrophy (replacement of muscle with connec-
tive tissue and fat) occurs. Microtubules perform vesicular and organelle
transport functions (answer d),and intermediate filaments, not actin, form
the intracellular connection in desmosomes (answer e).
130.The answer is a.(Alberts, pp 184, 952, 953, 972.)DLM’s peripheral
blood PMNs have increased activity as measured by oxidative burst and
F-actinassays. Actin maintains the mechanical strength of the cytoplasm of
the cell and is essential for cellular functions that require surface motility.
Those functions include phagocytosis, cytokinesis, and cell locomotion.
Although movement of vesicles along filaments is regulated by minimyosins
(myosin I), movement of vesicles and organelles is predominantly a func-
tion of microtubules (answers b and c)under the influence of the unidi-
rectional motors kinesin and dynein. The movements of chromosomes
(answer d)as well as the cilia and flagella (answer e)are driven by dynein,
and chromosomal movements occur through microtubular kinetics.
Muscle and Cell Motility Answers 225