Abstract
Dynamic remodelling of actin stress fibres (SFs) allows non-muscle cells to adapt to applied forces such as uniaxial cell shortening. However, the mechanism underlying rapid and selective disassembly of SFs oriented in the direction of shortening remains to be elucidated. Here, we investigated how myosin crossbridge cycling induced by MgATP is associated with SF disassembly. Moderate concentrations of MgATP, or [MgATP], induced SF contraction. Meanwhile, at [MgATP] slightly higher than the physiological level, periodic actin patterns emerged along the length of SFs and dispersed within seconds. The actin fragments were diverse in length, but comparable to those in characteristic sarcomeric units of SFs. These results suggest that MgATP-bound non-muscle myosin II dissociates from the individual actin filaments that constitute the sarcomeric units, resulting in unbundling-induced disassembly rather than end-to-end actin depolymerization. This rapid SF disassembly occurred independent of dephosphorylation of myosin light chain. In terms of effects on actin-myosin interactions, a rise in [MgATP] is functionally equivalent to a temporal decrease in the total number of actin-myosin crossbridges. Actin-myosin crossbridges are known to be reduced by an assisting load on myosin. Thus, the present study suggests that reducing the number of actin-myosin crossbridges promotes rapid and orientation-dependent disassembly of SFs after cell shortening.
Highlights
Actin stress fibres (SFs) are contractile structures in nonmuscle cells [1]
We observed that periodic actin patterns emerged along the length of SFs, and the pattern disappeared during rapid SF disassembly caused by high [MgATP]
Fluorescence microscopy shows that the shed actin fragments had lengths comparable to those in the sarcomeric units of SFs, suggesting that unbundling of SFs is responsible for the disassembly
Summary
Actin stress fibres (SFs) are contractile structures in nonmuscle cells [1]. SFs are primarily composed of actin, non-muscle myosin II (NMII), a-actinin and tropomyosin [2]. The structure of mature SFs has periodic arrays of these proteins similar to the organization of myofibrils. The periodicity measured in non-muscle PtK2 cells is approximately 1.2 mm [3]. Contractile forces generated by NMII–actin interactions can lead to shortening of existing SFs, as well as contribute to the assembly of the extracellular matrix [4,5], focal adhesions and SFs themselves [2,6,7,8,9,10].
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