The goal of the Voorhees lab is to identify and characterize the machinery that governs the biogenesis and quality control of membrane proteins. We do this using a combination of structural and functional strategies, including in vitro reconstitution, biochemical and cell based assays, and cryo-electron microscopy (cryo-EM).
Membrane protein biogenesis
One class of proteins that is particularly dependent on exogenous factors for their biogenesis are secreted and integral membrane proteins. This family of proteins make up ~30% of the eukaryotic proteome, and is essential for a range of cellular functions including intracellular trafficking, cell signaling, and the transport of molecules across the lipid bilayer. Defects in membrane protein maturation underlie numerous protein misfolding diseases, and more than half of all therapeutic drugs bind a membrane protein target. The essential roles of these proteins, as well as the consequences of their failed maturation, underscore the physiologic importance of understanding the molecular details of membrane protein biogenesis.
Both secreted and integral membrane proteins contain one or more hydrophobic segments that must be inserted into the lipid bilayer in the correct orientation for folding and function of the final protein. These assembly steps occur at the ER, where the majority of proteins are co-translationally translocated or inserted by the universally conserved Sec61 channel.
However, the Sec61 channel alone is sufficient for translocation of only a small number of model substrates. Indeed the majority of secreted and integral membrane proteins require additional factors for their modification, insertion, and folding. Despite the critical role of these proteins, very little is known about their overall architecture, interaction with the nascent polypeptide and signal sequence, or in some cases, even their role in translocation.
Our lab uses genetic, biochemical, and structural approaches to understand how the enormous diversity of secreted and multi-pass membrane proteins are assembled in membranes throughout eukaryotic cells.
Membrane protein insertion by the human ER membrane protein complex (EMC)
One of the definitive steps in membrane protein biogenesis is the insertion of their hydrophobic transmembrane domains (TMDs) into the bilayer. Because membrane proteins differ substantially in their topological and biophysical features, they require multiple parallel pathways to mediate their insertion into the bilayer. The ER membrane protein complex (EMC), a broadly conserved eukaryotic macromolecular machine, has emerged as an essential player in the biogenesis of a diverse set of membrane protein substrates. The EMC functions in both the co- and post-translational insertion of TMDs into the ER, and also plays a role as an intramembrane chaperone. Clients of the EMC therefore include many tail-anchored proteins, GPCRs, as well as the membrane proteins of many enveloped viruses including SARS-CoV-2.
Recently, our lab determined the structure of the human EMC using single particle cryo-electron microscopy (cryo-EM), which provided molecular insight into how the EMC captures substrate TMDs from the cytosol and catalyzes their insertion into the bilayer. Given the diversity, breadth, and physiologic importance of EMC substrates, the function of the EMC has broad biologic and biomedical implications.
Membrane protein insertion into mitochondria
Mitochondria are essential eukaryotic organelles that play critical roles in cellular metabolism and signaling. Both mitochondrial dynamics and communication with the host cell are mediated by proteins embedded in the outer mitochondrial membrane. In mammals, this includes ~150 proteins with alpha-helical TMDs that regulate processes such as apoptosis and the innate immune response. Despite their physiologically critical role, how these alpha-helical proteins are inserted into mitochondria was not understood.
In collaboration with the Weissman lab at MIT/Whitehead, we recently identified MTCH2 as a long-sought insertase for alpha-helical proteins into mitochondria. We showed that MTCH2 is both necessary and sufficient for insertion of diverse membrane protein substrates in cells and in vitro. MTCH2 exploits an ancestral transporter fold and is the defining member of new class of membrane protein insertases conserved across holozoa. Understanding MTCH2's function provides a biochemical rationalization for its pleotropic phenotypes and association with Alzheimer's disease and leukemia. This work establishes a critical genetic and biochemical toolset for future questions probing the biogenesis and quality control of mitochondrial membrane proteins.