Membrane Protein Complexes: Structure and Function

Michael A. Harrison, Steven P. Muench, Harrison, Michael A., Muench, Steven P.

The vacuolar H+-ATPase (V-ATPase) is a ~1 MDa membrane protein complex that couples the hydrolysis of cytosolic ATP to the transmembrane movement of protons. In essentially all eukaryotic cells, this acid pumping function plays critical roles in the acidification of endosomal/lysosomal compartments and hence in transport, recycling and degradative pathways. It is also important in acid extrusion across the plasma membrane of some cells, contributing to homeostatic control of cytoplasmic pH and maintenance of appropriate extracellular acidity. The complex, assembled from up to 30 individual polypeptides, operates as a molecular motor with rotary mechanics. Historically, structural inferences about the eukaryotic V-ATPase and its subunits have been made by comparison to the structures of bacterial homologues. However, more recently, we have developed a much better understanding of the complete structure of the eukaryotic complex, in particular through advances in cryo-electron microscopy. This chapter explores these recent developments, and examines what they now reveal about the catalytic mechanism of this essential proton pump and how its activity might be regulated in response to cellular signals.