Affinity of PIP-aquaporins to sterol-enriched domains in plasma membrane of the cells of etiolated pea seedlings

The hypothesis that sterol-enriched domains represent sites of preferred localization of PIP-aquaporins was tested in experiments on plasma membranes isolated from cells of etiolated pea (Pisum sativum L.) seedlings. Plasma membranes were isolated from microsomes by the partition in the aqueous two-phase polymer system and separated into vesicle fractions of different buoyant density by flotation in discontinuous OptiPrep gradient. Two types of plasma membrane preparations were used: one was treated with cold 1% Triton X-100 and the other was not. In untreated preparations, three populations of plasma membrane vesicles were obtained, while in the case of treated preparations, fractions of detergent-resistant membranes (DRM) and solubilized membrane proteins were obtained. In all membrane fractions collected after OptiPrep flotation, the amounts of proteins, sterols, and PIP-aquaporins were determined. The highest sterol content was detected in the membrane fraction with buoyant density 1.098 g/cm³ and in the DRM fraction (1.146 g/cm³). These fractions contained much more PIP-aquaporins than the other ones. Phase state of the lipid bilayer was determined by measuring generalized polarization excitation of fluorescence (GPEX) of laurdan incorporated into the membranes of different fractions. It was revealed that the lipid bilayer of the membranes with density of 1.098 g/cm³ had a higher extent of ordering than that of the fractions with density of ∼1.146 g/cm³. The results indicated that uppermost local concentrations of PIP-aquaporins were associated with tightly packed sterol-enriched domains. Moreover, upon solubilization of plasma membrane with Triton X-100, PIP-aquaporins mainly resided in DRM, thus exhibiting a high affinity to sterols.