A nebulin ruler does not dictate thin filament lengths.
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 mum from the Z-line, but Tmod localized 1.13-1.31 mum 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.
Castillo
Angelica
A
Department of Forensic Science, Chaminade University, Honolulu, Hawaii, USA.
Nowak
Roberta
R
Littlefield
Kimberly P
KP
Fowler
Velia M
VM
Littlefield
Ryan S
RS
D000199:Actins / Q000378:metabolism; D000818:Animals; D002645:Chickens; D005260:Female; D005455:Fluorescent Antibody Technique; D008297:Male; D009124:Muscle Proteins / Q000737:chemistry / Q000378:metabolism*; D054792:Muscle, Striated / Q000378:metabolism* / Q000648:ultrastructure; D009210:Myofibrils / Q000378:metabolism* / Q000648:ultrastructure; D011817:Rabbits; D051348:Tropomodulin / Q000737:chemistry / Q000378:metabolism
D000199:Actins; D009124:Muscle Proteins; D051348:Tropomodulin; C035620:nebulin