protein. In the center of the A band is a lighter-staining area that consists
only of thick (rod-like portions of myosin) filaments and is known as the H
band (Don the TEM). Lateral connections occur between adjacent thick
filaments in the region of the M line (Bon the TEM), which bisects the H
zone and is composed primarily of creatine kinase, an enzyme that catalyzes
the formation of ATP.
133.The answer is d.(Alberts, pp 953, 959, 979, 1037. Junqueira, pp
57–62.)Kinesin moves vesicles unidirectionally from the minus end to the
plus end of the microtubule—for example, from the cell body to the axon
terminus in fast axonal transport. Myosin, minimyosin, dynein, dynamin,
and kinesin (answers a, b, c, and e)are all mechanochemical enzymes or
molecular “motors” that hydrolyze ATP and undergo conformational
changes that are converted into movement. Cytoplasmic dynein is respon-
sible for movement toward the minus end of the microtubules. Remember
kinesinkicks out and dyneindrags them in (see feedback to question 41).
Ciliary and flagellar bending is the classic model for microtubule-based
motility. The motor is dynein, which causes the relative sliding between
microtubules in the axoneme. Structural constraints within the axoneme as
a whole convert sliding into ciliary bending.
Dynamin is another ATPase motor that mediates sliding between adja-
cent cytoplasmic microtubules.
Filamin or other actin cross-linking proteins form a gel network in the cell
cortex (the area just beneath the cell membrane). The presence of the actin gel
in the cell cortex contributes to the rigidity of the cell and is also involved in
changes in cell shape and chromosome movements during mitosis.The sar-
comere extends from Z disc (line) to Z disc. The H band is located between the
ends of the thin (actin) filaments. The A band is defined by the width of the
thick (myosin) filaments. The I bands are not shown completely on the figure
because they are found between adjacent A bands.
Muscle and Cell Motility Answers 227