THE 17 KDA BAND IDENTIFIED BY MULTIPLE ANTI-AQUAPORIN-2 ANTISERA IN RAT-KIDNEY MEDULLA IS A HISTONE

The osmotic water permeability of epithelial cells of the inner medull ary collecting duct of the kidney is regulated by antidiuretic hormone (ADH). ADH causes the insertion and removal of cytoplasmic vesicles c ontaining the aquaporin (AQP-2) water channel protein which is recogni zed by multiple rabbit antipeptide antisera raised against amino acid sequences comprising its cytoplasmic carboxyl terminal. Immunoblots of rat kidney membrane fractions as well as human urine have all shown t hat AQP-2 is expressed exclusively by collecting duct cells and have i dentified a 29 kDa band (corresponding to the nonglycosylated AQP-2 pr otein), a broad 35-45 kDa band (corresponding to the mature glycosylat ed form of AQP-2 protein) and an additional immunoreactive 17 kDa band of unknown origin. We now report that the 17 kDa band identified by t hese anti-AQP-2 antisera is not an AQP-2 component but rather a denatu red histone protein type H2A1, This binding of anti-AQP-2 antisera to denatured H2A1 present in protein samples derived from both kidney inn er medulla and human urine is blocked specifically by preincubation of immunoblots with solutions containing the acidic protein gelatin.