IMPORTANCE OF THE MERCURY-SENSITIVE CYSTEINE ON FUNCTION AND ROUTING OF AQP1 AND AQP2 IN OOCYTES

To discriminate between water transport of of aquaporin-2 (AQP2) mutan ts in nephrogenic diabetes insipidus and that of an AQP2 molecule used to drag them to the oolemma, we investigated the mercury sensitivity of wild-type and AQP2 C181S proteins in oocytes. Incubation with HgCl2 inhibited the osmotic water permeability (P-f) of human (h) AQP2 by 4 0%, whereas inhibition of hAQP1 was 75%. Oocytes expressing hAQP1 C189 S revealed a P-f comparable to wild-type hAQP1, but mercury sensitivit y was lost. In contrast, no increase in P-f was obtained when hAQP2 C1 81S was expressed. Also, expression of rat AQP2 C181A and C181S mutant s did not increase the P-f, which contrasts with published observation s. Immunocytochemistry and immunoblotting revealed that only AQP1, AQP 1 C189S, and AQP2 were targeted to the plasma membrane and that AQP2 m utant proteins are retarded in the endoplasmic reticulum. In conclusio n, water transport through AQP2 is less sensitive to mercury inhibitio n than through AQP1. Furthermore, substitution of the mercury-sensitiv e cysteine for a serine results in an impaired routing of human and ra t AQP2. Similar mutations have no effect on AQP1 function, which is in dicative of structural differences between AQP1 and AQP2.