Research in the Stevens lab is concerned with the process of protein sorting and membrane assembly in yeast cells. Using yeast molecular genetics, we have identified a large number of genes required for the correct targeting and transport of proteins to the membrane-bounded organelle called the vacuole. These vacuolar protein sorting (VPS) genes have been found to encode proteins such as a dynamin-like GTPase, a protein-sorting receptor, a protein kinase, a lipid kinase, a RAS inhibitor-like protein, and an increasingly large number of proteins involved in transport vesicle targeting/fusion such as Rab-like GTPases and SNARE proteins. To characterize the function of some of these proteins we use biochemical, cell biological and molecular genetic approaches. Biochemical approaches are being used to isolate a number of the VPS proteins and to study the membrane-associated protein complexes in which they are found.
The group also has a long-standing interest in the assembly, targeting, structure and function of the vacuolar H+-translocating ATPase (V-ATPase; see figure). The V-ATPase complex consists of fourteen subunits, and all but one of these are encoded by a single yeast gene. The large hydrophobic V-ATPase “a” subunit has two isoforms, Vph1 and Stv1, with the Vph1-associated V-ATPase complex localizing to the vacuole membrane and the Stv1-associated V-ATPase restricted to Golgi and endosomal membranes. The mechanism of differential localization of these two forms of the yeast V-ATPase is under active investigation in the lab. We are investigating the proteins responsible for maintaining this differential localization, as well as the protein-based signals that specify the distinct localizations.
We have also identified five genes that encode proteins required for V-ATPase complex assembly but are not themselves part of the final V-ATPase enzyme complex. These five proteins reside in the yeast cell endoplasmic reticulum and constitute the dedicated assembly machinery for the V-ATPase. A number of molecular genetic and biochemical approaches are being taken to characterize the assembly complex and to study the interaction of this assembly complex with V-ATPase subunits along the assembly pathway within the endoplasmic reticulum.
The lab has also employed ancestral gene reconstruction to investigate the V-ATPase enzyme complex in more detail. Ancestral reconstruction of the two V-ATPase subunit a isoforms has generated the most likely predecessor gene prior to gene duplication. This Anc.a protein functions with the remaining 13 V-ATPase subunits and has characteristics of both the Stv1- and Vph1-containing V-ATPase complexes. Ancestral reconstruction of the most likely predecessor of the Vma3 and Vma11 proteolipid V-ATPase subunits has led to the synthesis of an Anc.3-11 the functions with the remaining 12 V-ATPase subunits to form a functional enzyme complex. The Anc.3-11 protein allows the assembly of a proteolipid ring with two different polypeptides (Anc.3-11 and Vma16) rather than the modern-day ring with three different polypeptides (Vma3, Vma11 & Vma16). These investigations have revealed important insights into the modern-day V-ATPase complex as well as the evolution of this V-ATPase molecular machine.
(pulled from pubmed)