Dynamics of actomyosin filaments in the contractile ring revealed by ultrastructural analysis

Abstract

Cytokinesis, the final process of cell division, involves the accumulation of actin and myosin II filaments at the cell's equator, forming a contractile ring that facilitates the division into two daughter cells. While light microscopy has provided valuable insights into the molecular mechanism of this process, it has limitations in examining individual filaments in vivo. In this study, we utilized transmission electron microscopy to observe actin and myosin II filaments in the contractile rings of dividing Dictyostelium cells. To synchronize cytokinesis, we developed a novel method that allowed us to visualize dividing cells undergoing cytokinesis with a frequency as high as 18%. This improvement enabled us to examine the lengths and alignments of individual filaments within the contractile rings. As the furrow constricted, the length of actin filaments gradually decreased. Moreover, both actin and myosin II filaments reoriented perpendicularly to the long axis during furrow constriction. Through experiments involving myosin II null cells, we discovered that myosin II plays a role in regulating both the lengths and alignments of actin filaments. Additionally, dynamin‐like protein A was found to contribute to regulating the length of actin filaments, while cortexillins were involved in regulating their alignment.

Suggesting Dictyostelium as a Model for Disease-Related Protein Studies through Myosin II Polymerization Pathway

“…These data demonstrate a greater impact of ATP on filament width. The polymerized filament lengths are consistent with measurements from total internal reflection fluorescence (TIRF) microscopy (680 nm, 400 to 950 nm) and thin-section TEM (500 nm, 400 to 600 nm) in our experimental conditions [24,25]. Filaments longer than 600 nm in thin-section TEM observation [20] could be missed due to disassembly during fixation [26].…”

“…Filament lengths range from 0.4 to 0.95 µm, with a median length of 0.68 µm, consistent with the heterogeneity observed in vitro. Filament lengths measured in situ using TIRF microscopy are notably longer than those obtained with thin-section EM, which showed a length of 0.5 µm [25,26].…”