A Nebulin Ruler Does Not Dictate Thin Filament Lengths |
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Authors: | Angelica Castillo Kimberly P Littlefield Ryan S Littlefield |
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Institution: | † Department of Forensic Science, Chaminade University, Honolulu, Hawaii ‡ Department of Cell Biology, The Scripps Research Institute, La Jolla, California § Center for Cell Dynamics, University of Washington, Friday Harbor Laboratories, Friday Harbor, Washington |
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Abstract: | To generate force, striated muscle requires overlap between uniform-length actin and myosin filaments. The hypothesis that a nebulin ruler mechanism specifies thin filament lengths by targeting where tropomodulin (Tmod) caps the slow-growing, pointed end has not been rigorously tested. Using fluorescent microscopy and quantitative image analysis, we found that nebulin extended 1.01-1.03 μm from the Z-line, but Tmod localized 1.13-1.31 μm from the Z-line, in seven different rabbit skeletal muscles. Because nebulin does not extend to the thin filament pointed ends, it can neither target Tmod capping nor specify thin filament lengths. We found instead a strong correspondence between thin filament lengths and titin isoform sizes for each muscle. Our results suggest the existence of a mechanism whereby nebulin specifies the minimum thin filament length and sarcomere length regulates and coordinates pointed-end dynamics to maintain the relative overlap of the thin and thick filaments during myofibril assembly. |
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