CHARACTERISTICS OF HG-SENSITIVE AND ZN-SENSITIVE WATER CHANNELS IN THE PLASMA-MEMBRANE OF CHARA CELLS

Hydraulic conductivity (L(p)) of the plasma membrane of Chara corallin a was inhibited by HgCl2 maximally by about 95%. The inhibition was re versed by 2-mercaptoethanol, reconfirming the observation obtained by Henzler and Steudle (1995). The results suggest that osmotic water tra nsport through Chore cells occurs mostly via mercury-sensitive water c hannels 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 Hep es-Tris buffer (pH 7.4) inhibited it by about 90% within several minut es. inhibition of L(p) by ZnCl2 was also reversed by 2-mercaptoethanol , suggesting that zinc acts also on thiol groups of water channel prot eins. Cells from which tonoplast had been removed by EGTA were as sens itive to both HgCl2 and ZnCl2 (pH 7.4) as normal cells. This demonstra tes that water channels sensitive to thiol reagents really exist in th e 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 fir st that Hg- and Zn-sensitive thiol groups of water channels may exist on the cytoplasmic side, and second that ZnCl2 in acidic medium may ex ist in ionized species which can be chelated by EGTA after permeation. The polar water permeability, or the endoosmotic L(p) being larger th an the exoosmotic one, was not affected by lowering the rate of osmosi s by decreasing the osmotic gradient for transcellular osmosis down to 0.02 M sorbitol. The polarity disappeared when osmotic water flow thr ough water channels was completely inhibited by HgCl2. Thus the polari ty is assumed to be intrinsic to water channels in the plasma membrane .