Characteristics of Hg- and Zn-sensitive water channels in the plasma membrane of Chara cells

Hydraulic conductivity (L(p)) of the plasma membrane of Chara corallina was inhibited by HgCl2 maximally by about 95%. The inhibition was reversed by 2-mercaptoethanol, reconfirming the observation obtained by Henzler and Steudle (1995). The results suggest that osmotic water transport through Chore cells occurs mostly via mercury-sensitive water channels containing thiol groups. ZnCl2 dissolved in APW (pH 5.6) also inhibited L(p) by about 80% within 1-2 h, while ZnCl2 dissolved in Hepes-Tris buffer (pH 7.4) inhibited it by about 90% within several minutes. inhibition of L(p) by ZnCl2 was also reversed by 2-mercaptoethanol, suggesting that zinc acts also on thiol groups of water channel proteins. Cells from which tonoplast had been removed by EGTA were as sensitive to both HgCl2 and ZnCl2 (pH 7.4) as normal cells. This demonstrates that water channels sensitive to thiol reagents really exist in the plasma membrane. On the other hand, ZnCl2 (pH 5.6) did not inhibit L(p) of tonoplast-free cells. This may be accounted for by assuming first that Hg- and Zn-sensitive thiol groups of water channels may exist on the cytoplasmic side, and second that ZnCl2 in acidic medium may exist in ionized species which can be chelated by EGTA after permeation. The polar water permeability, or the endoosmotic L(p) being larger than the exoosmotic one, was not affected by lowering the rate of osmosis by decreasing the osmotic gradient for transcellular osmosis down to 0.02 M sorbitol. The polarity disappeared when osmotic water flow through water channels was completely inhibited by HgCl2. Thus the polarity is assumed to be intrinsic to water channels in the plasma membrane.