Oligomerization of water and solute channels of the major intrinsic protein (MIP) family

Abstract

Water and small solute fluxes through cell membranes are ensured in many tissues by selective pores that belong to the major intrinsic protein family (MIP). This family includes the water channels or aquaporins (AQP) and the neutral solute facilitators such as the glycerol facilitator (GlpF). We have compared the characteristics of representatives of each subfamily. Following solubilization in the nondenaturing detergents n-octyl-glucoside (OG) and Triton X-100 (T-X100), AQPs remain in their native homotetrameric state, while GlpF always behaves as a monomer. Solute facilitators are fully solubilized by the detergent N-lauroyl sarcosine (NLS), while AQPs are not. Analyses of mutants and chimeras demonstrate a close correlation between the water transport function and the resistance to NLS solubilization. Thus, AQPs and solute facilitators exhibit different behaviors in mild detergents; this could reflect differences in quaternary organization within the membranes. We propose that the oligomerization state or the strength of self-association is part of the mechanisms used by MIP proteins to ensure solute selectivity.