Cell motility is one of the most attractive phenomena among cell activities, and variety types of motility are observed in protozoa. I have studied on the mechanism of amoeboid movement, employing glycerinated model, reconstitution model, analysis of membrane dynamics and compression model. The data demonstrates that the dominant motive force of the movement is the hydrostatic pressure which was generated by the contraction of actomyosin layer in the subcortical cytoplasm. The plasma membrane is pushed by the pressure and extends forward by unfolding the wrinkles of the cell surface at the rear region. The smooth extension of the plasma membrane is achieved by its high fluidity.
Various types of adherent eukaryotic cells perform amoeboid locomotion. Until recently, it had been thought that the motive force of the locomotion was mainly produced by polymerization of actin at leading edge. However, more recently, another mechanism has been proposed. The mechanism is called bleb-driven amoeboid locomotion, typically seen in the locomotion of a giant-free-living amoeba, Amoeba proteus. The motive force of the locomotion is intracellular hydrostatic pressure, which is produced by contraction of the cell cortex actomyosin. In this review, we describe the mechanism of bleb-driven amoeboid locomotion through recent studies.
Ciliate conjugation usually occurs when cells are moderately starved after rapid proliferation. In Blepharisma japonicum, at the stationary phase, a mating pheromone (gamone 1) is secreted by mating type I cells and stimulates mating type II cells to secrete another mating pheromone (gamone 2), which stimulates mating type I cells. Both stimulated cells are transformed to be able to form pairs. We showed previously that the gamone 1 transcription occurs only in the stationary-phase cells, not in logarithmic-phase cells. However, it is still unknown how rapidly cells response to starvation and express gamone 1. Here we show that gamone 1 expression is initiated by sudden artificial starvation, and we determined the time required for the initiation of transcription and secretion of gamone 1 in B. japonicum. Proliferating cells rapidly responded to artificial starvation, approx. 1.5 h at most was necessary for the initiation of the transcription of gamone 1 gene, and approx. 3 h was needed for the secretion of gamone 1. Gamone 1 expression tested in single cells suggests that gamone 1 expression may be correlated with the cell-cycle, most probably with the certain stage of G1 phase in B. japonicum. Our present findings provide an experimental system to investigate the environmental factors involved in the initiation of conjugation.