Stretch-activated Ca2+ and Ca2+-activated K+ channels in the hyphal tip plasma membrane of the oomycete Saprolegnia ferax

Abstract

We have used the patch clamp technique to study ion channels involved in hyphal tip growth in the oomycete Saprolegnia ferax. The development of a technique in which protoplasts are produced in distinct linear arrays has permitted the study of transport proteins in protoplasts derived from different regions of the hypha. Using the cell-attached mode we find two K⁺ channels of different amplitudes, both of which are activated by Ca²⁺, as shown by the addition of the Ca²⁺ ionophore A23187. In nearly all the recordings (95%) there was a characteristic but variable oscillatory nature to the activity of these channels. K⁺ fluxes through these channels are inward and are of a sufficient magnitude to have a significant impact on cell turgor. Hyphal tip, but rarely distal protoplasts, also contain two stretch-activated channels, one displaying larger amplitudes and permeable to both Ca²⁺ and K⁺ and a smaller channel permeable to Mg²⁺. Stretch-activated ion channels permeable to calcium are of particular interest in relation to tip growth and their concentration at the hyphal tip supports a direct role in the tip growth process. This represents a significant advance toward understanding the mechanisms of regulation of this mode of cellular growth.