Analysis of Soluble Sugar Permease Domains by Solution NMR

Knowledge of membrane transporter molecular structures is crucial for obtaining a detailed understanding of the mechanism by which these proteins shuttle their cargo across a biological membrane. Unfortunately, ascertaining complete structures of these transporters currently poses a difficult challenge for the structural biologist, as whole membrane proteins are generally refractory to analyses by X-ray crystallography or nuclear magnetic resonance (NMR) spectroscopy. The hydrophobic nature of these proteins leads to aggregation that represents a barrier to crystallization; similarly, the aggregation properties of these proteins in solution thwart efforts to collect high-quality NMR spectra. Nonetheless, NMR spectroscopy does provide some experimental avenues for characterizing membrane transporter structure and function. Membrane transporters frequently take on a modular structure, being organized into cytoplasmic and integral membrane domains that perform distinct functions. The soluble cytoplasmic domains of a number of sugar permeases have been studied extensively using various solution NMR techniques (1 –6 ), and NMR methodologies applied toward such studies will be described in this chapter. In addition, solid-state NMR has been employed to examine the conformation of transmembrane segments in lipid environments; descriptions of such studies are provided elsewhere in this volume (see Chapter 16 ).