A researcher is investigating the structure of the sarcomere using skeletal muscle obtained from an experimental animal. He develops monoclonal antibodies directed against a specific skeletal muscle protein and finds that these antibodies disrupt the binding of actin to structural support elements within the sarcomere. Electron microscopy of the sarcomere is shown in the image below.
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Which of the following labeled regions do these antibodies most likely bind?
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A sarcomere is composed of overlapping filaments of actin and myosin as well as structural and binding proteins (eg, titin, α-actinin). A single sarcomere is defined as the distance between two Z lines. Thin (actin) filaments are bound to structural proteins at the Z line. The unbound ends of the actin filaments project into the middle of the sarcomere, where they interact with thick (myosin) filaments during muscle contraction.
(Choice B) The A band corresponds to myosin filaments in the sarcomere. In this region, there are myosin filaments overlapped with actin filaments as well as non-overlapped myosin filaments. The A band always remains the same length during muscle contraction.
(Choice C) The M line lies at the center of the A band and is where myosin filaments anchor to structural elements in the center of the sarcomere.
(Choice D) Myosin and actin filaments overlap at the segment of the sarcomere between the H and I bands.
(Choices E and F) The I band is the region of the sarcomere in which actin does not overlap with myosin, and the H band is the region in which myosin does not overlap with actin. During muscle contraction, both H and I bands decrease in length.
Educational objective:
A single sarcomere is defined as the distance between two Z lines. Thin (actin) filaments in the I band are bound to structural proteins at the Z line, whereas thick (myosin) filaments in the A band are bound to structural proteins at the M line.