Oligomerization and folding in membrane proteins
We are interested in how the primary sequences of membrane proteins determine their three dimensional structures, and hence their functions. The folding of integral membrane proteins clearly differs from that of soluble proteins since the membrane environment imposes constraints on polypeptide secondary and tertiary structural features quite different from those imposed by an aqueous environment.
A conceptual underpinning for much of the work in the group is that, for helical transmembrane proteins, the protein folding process can be considered to occur in two kinetically separated and therefore energetically distinct stages. First, transbilayer alpha-helices are formed (stage I) and second, the helices interact within the bilayer to form a specific globular tertiary structure (stage II).
In the case of oligomerization events, monomeric proteins are synthesized and inserted into the membrane, and these monomers subsequently interact in a side-to-side fashion to form complexes that involve helix-helix interactions similar to those found within polytopic helical membrane proteins.
Our most recent work in this area is to examine the association of transmembrane domains (TMs) involved in signaling by receptors that have a single TM, where the signaling mechanism is mediated by TM interaction.
Use and mechanism of pH dependent TM insertion
We have previously observed the spontaneous, pH-dependent insertion of a water-soluble peptide to form a helix across lipid bilayers. We have now used a related peptide, pH (low) insertion peptide (pHLIP), to translocate cargo molecules attached to its C terminus across the plasma membranes of living cells. Translocation is selective for low pH, and various types of cargo molecules attached by disulfides can be released by reduction in the cytoplasm, including peptide nucleic acids, a cyclic peptide (phalloidin), and organic compounds. Because a high extracellular acidity is characteristic of a variety of pathological conditions (such as tumors, infarcts, stroke-afflicted tissue, atherosclerotic lesions, sites of inflammation or infection, or damaged tissue resulting from trauma), the pH (low) insertion peptide may prove a useful tool for selective delivery of agents for drug therapy, diagnostic imaging, genetic control, or cell regulation.
We have recently shown that pHLIP can localize and map acidic foci in kidneys, tumors and inflammatory sites in vivo. In a mouse breast adenocarcinoma model, fluorescently labeled pHLIP finds solid acidic tumors with high accuracy and accumulates in them even at a very early stage of tumor development. The peptide has three states: soluble in water, bound to the surface of a membrane, and inserted across the membrane as an alpha-helix. At physiological pH, the equilibrium is toward water, which explains its low affinity for cells in healthy tissue; at acidic pH, titration of Asp residues shifts the equilibrium toward membrane insertion and tissue accumulation. The pHLIP technology introduces a new concept to detect, target, and possibly treat acidic diseased tissue by employing the selective insertion and folding of membrane peptides.
We are continuing our work on the fundamental mechanism and capabilities of the pHLIP technology.