Abstract
Contractile function of striated muscle cells depends crucially on the almost crystalline order of actin and myosin filaments in myofibrils, but the physical mechanisms that lead to myofibril assembly remains ill-defined. Passive diffusive sorting of actin filaments into sarcomeric order is kinetically impossible, suggesting a pivotal role of active processes in sarcomeric pattern formation. Using a one-dimensional computational model of an initially unstriated actin bundle, we show that actin filament treadmilling in the presence of processive plus-end crosslinking provides a simple and robust mechanism for the polarity sorting of actin filaments as well as for the correct localization of myosin filaments. We propose that the coalescence of crosslinked actin clusters could be key for sarcomeric pattern formation. In our simulations, sarcomere spacing is set by filament length prompting tight length control already at early stages of pattern formation. The proposed mechanism could be generic and apply both to premyofibrils and nascent myofibrils in developing muscle cells as well as possibly to striated stress-fibers in non-muscle cells.
Highlights
The intriguing striations of muscles were first observed more than a century ago [1]
Actin filaments with nearby plus-ends can form a stable crosslink by a complex of molecules that holds the plus-end of the two actin filaments, but still allows for actin polymerization at the plus-end, see section ‘The computational model’ and figure 2A
Each actin cluster will move as a whole subject to the sum of forces acting on its constituent actin filaments
Summary
The intriguing striations of muscles were first observed more than a century ago [1]. Spaced crosslinking regions termed Z-bodies or Z-bands delineate 1mm-wide sarcomeric regions that comprise actin filaments of organized polarity and crosslinking myosin filaments in the sarcomere midzone How are these surprisingly regular structures assembled? Nascent striations first become visible as agglomerations of the actin crosslinker a-actinin, which grow and change position to establish a regular, periodic spacing [10] The formation of these early, unstriated bundles requires the parallel alignment of actin filaments, their mutual crosslinking as well as some means to control bundle thickness. We ask if physical interactions of actin and myosin filaments, as well as crosslinkers are sufficient to induce initial striated order in filament bundles Such a mechanism could be generic and could apply to the formation of striations in acto-myosin stress fibers in
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