THE TOUCH-INDUCED ACTION-POTENTIAL IN CHARA - INQUIRY INTO THE IONIC BASIS AND THE MECHANORECEPTOR

We developed an assay to characterize the mechanoreceptor for the touc h-induced action potential of Chara internodal cells in order to compa re it with the mechanoreceptor involved in sensing the gravitational p ressure that induces a polarity of cytoplasmic streaming in the same c ells. We show that there are at least two classes of mechanoreceptors in characean internodal cells. One class contains a RGD-binding, integ rin-like protein that is localized at the end of the cells. This class of mechanoreceptors requires a turgid plasma membrane for sensing-com petence and does not exhibit adaptation. We propose that this type of mechanoreceptor senses gravitational and hydrostatic pressure by sensi ng the magnitude and sign of its displacement relative to the extracel lular matrix. The second class of mechanoreceptors most likely does no t contain RGD-binding, integrin-like proteins and does not require a t urgid plasma membrane. This class is uniformly localized throughout th e length of the cell and displays adaptation. We propose that this cla ss of mechanoreceptors senses touch by sensing changes in tension whic h is a result of the magnitude but not the sign of the displacement of the membrane.