A MECHANISTIC INTERPLAY BETWEEN AMPHIPHILIC BILAYER AND INTEGRAL CHANNEL PROTEIN AQPZ

Abstract

An emerging area in the recent study of the membrane channel proteins is that the emphasis on the bilayer membrane, which was once considered as a passive cellular component, can in fact play a highly active role in modulating the channel proteins? structures and/or activity. Therefore it has become an imperative to gain insights about the fine interplay between the bilayer and the embedded integral membrane protein. The integral membrane protein of our interest is Aquaporin-Z (Aqp-Z), a novel, robust and dedicated water channel protein derived from Escherichia coli. We have employed different methods to effectively incorporate Aqp-Z into both the native cellular bilayer (e.g., PC and PE lipids) and the synthetic amphiphilic bilayer (e.g., diblock copolymer bilayer). The experimental work has clearly indicated that Aqp-Z exhibits distinctive activities when being incorporated into the bilayers of the extensive nature. Subsequently, we have categorized the bilayers by their biophysical factors: (1) the extent of unsaturation (the presence of C=C double bonds) in the bilayer; (2) the bilayer rigidity (the presence of cholesterol in the bilayer); (3) the hydrophobic and hydrophilic mismatch between the bilayer and Aqp-Z embedded in the bilayer, and systematically evaluated their contributions to the protein activity modification. Of the aforementioned, we ascribe this protein activity modification primarily to the hydrophilic/hydrophobic mismatch between the bilayer and Aqp-Z. In addition we have also attempted to study the work from the perspective of the coarse-grained MD simulation to further substantiate our findings. The lateral profile of the bilayer confirms that the drastic hydrophobic/hydrophilic mismatch between the synthetic bilayer and the Aqp-Z results in a rippling, perturbative effect in the membrane. This study presents a platform to elucidate the underlying water permeation mechanism through Aqp-Z, in particular the interaction between the Aqp-Z and the surrounding bilayer. It provides an additional mean to up-regulate the membrane protein activity, circumventing the problems of direct posttranslational modification on the proteins of delicate nature. Moreover, it also assists us in engineering the next generation water purification membrane.