1 Introduction

In recent years, analytical ultracentrifugation (AUC) has played an important role in the analysis of membrane proteins. Sedimentation velocity continues to be an extremely useful tool for assessing the homogeneity of membrane protein preparations in detergent micelle solutions; however, because of the multiple components present in a membrane protein:detergent micelle particle, the hydrodynamic interpretation of the sedimentation coefficient is not as straightforward as it is with soluble proteins. As in the past, sedimentation equilibrium continues to be used to determine the molecular weights of many membrane proteins when dissolved in detergent micelles. More recently, the method has played a major role in studies on the thermodynamics of subunit interactions between membrane proteins. The development of these protocols has spurned theories describing the dependence of a membrane protein oligomeric population on the amount of detergent as well as novel studies aimed at designing membrane proteins with desired oligomerization properties. The bulk of the contents of this chapter will therefore mainly focus on the issues related to measuring and interpreting the buoyant molecular weight of a membrane protein in complex with detergent.