Functional independence of monomeric CHIP28 water channels revealed by expression of wild-type mutant heterodimers.

CHIP28 is a major water transporting protein in erythrocytes and kidney which forms tetramers in membranes (Verbavatz, J. M., Brown, D., Sabolic, I., Valenti, G., Ausiello, D. A., Van Hoek, A. N., Ma, T., and Verkman, A. S. (1993) J. Cell Biol. 123, 605-618). To determine whether CHIP28 monomers function independently, chimeric cDNA dimers were constructed which contained wild-type CHIP28 in series with either wild-type CHIP28, a non-water transporting CHIP28 mutant (C189W), or a functional but mercurial-insensitive CHIP28 mutant (C189S). Transcribed cRNAs were injected in Xenopus oocytes and plasma membrane expression was assayed by quantitative immunofluorescence. Water channel function was measured by osmotically induced swelling. CHIP28 homo- and heterodimers were targeted to the oocyte plasma membrane and functioned as water channels. Relative osmotic water permeability (Pf) values (normalized for plasma membrane expression of monomeric subunits) were: 1.0 (CHIP28 monomer), 0.0 (C189W), 1.07 (C189S), 1.10 (CHIP28-CHIP28 dimer) and 0.52 (CHIP28-C189W). The increase in oocyte Pf was linearly related to plasma membrane expression of wild-type CHIP28 and C189S subunits. HgCl2 (0.3 mM) inhibited channel-mediated Pf in oocytes expressing wild-type CHIP28 monomers and dimers by 85-90%, but did not inhibit Pf in oocytes expressing C189S. HgCl2 inhibited Pf in oocytes expressing CHIP28-C189S dimers by 44 +/- 7%, consistent with one mercurial-sensitive and one insensitive subunit in the heterodimer. These results indicate that despite their assembly in tetramers, monomeric CHIP28 subunits function independently as water channels.