Characterization of human AQP5 mutations found in patients suffering from palmoplantar keratosis (Bothnian type).

Abstract

Aquaporins are structurally well conserved within all kingdoms of life and can be divided into two main groups: (i) orthodox aquaporins conducting only water molecules, and (ii) aquaglyceroporins channeling water and other small uncharged solutes. Water transport through these integral membrane channels takes place in a single-file fashion through the pores of each of the protomers that form tetrameric units in cell membranes.[PP1] Here we focus on the functional consequences of mutations in the orthodox aquaporin-5 (Aqp5), commonly expressed in salivary glands, digestive and respiratory systems, kidneys, the CNS, and sweat glands. These mutations cause hereditary palmoplantar keratoma (Bothnia type; PPKB), marked by hyperkeratotic lesions on the surface of palms and soles. They differ in the positions of the substituted amino acids as well as backbone interactions. We overexpressed some of the mutations in S. cerevisiae as YFP-fusion proteins, purified and reconstituted the water channels into lipid vesicles, and monitored osmotic vesicle deflation under stopped-flow conditions. Additionally assessing the number of reconstituted channels by fluorescence correlation spectroscopy allowed calculation of unitary water channel conductances. The significance of the alterations in water channeling capabilities is discussed with respect to disease genesis.