Dr. Capelluto’s current research centers on the molecular structure and biochemical functions of signaling transduction systems involved in membrane trafficking and cell signaling.
Our research goal is to understand how protein domains transduce signals from biological membranes. Our laboratory employs biophysical approaches including high field nuclear magnetic resonance spectroscopy, circular dichroism, computer modeling, liposome-binding assays, fluorescence spectroscopy and surface plasmon resonance spectroscopy. With these tools, we can determine protein:lipid interfaces, ligand binding pockets and membrane insertion of protein domains from the molecular to the atomic resolution.
Two major areas are currently developed in my group. The first area of research focuses in understanding how platelet aggregation is regulated. Platelets form a clump at the site of vascular injury to stop bleeding. One negative regulator of platelet aggregation is Disabled-2 (Dab2), a protein released to the extracellular surface upon platelet activation. Dab2 inhibits platelet aggregation through its phosphotyrosine-binding (PTB) domain by competing with fibrinogen for alphaIIb-beta3 integrin receptor binding. We have recently determined that N-terminal region of Dab2 that includes the PTB domain specifically binds sulfatides. Sulfatides are sphingolipids found on the platelet surface, which interact with coagulation proteins, playing a major role in haemostasis. One of them is P-selectin, a protein that promotes platelet-platelet and platelet-leukocyte interactions by binding to sulfatides at the target cell surface. Our results show that sulfatides recruit N-PTB to the platelet surface, sequestering it from integrin receptor binding during platelet activation. This is a transient recruitment that follows N-PTB internalization by an actin-dependent process. We demonstrate that sulfatide binding by N-PTB specifically inhibited P-selectin expression in activated platelets and P-selectin-mediated platelet-leukocyte aggregation. Our experimental data support a model where two pools of Dab2 co-exist at the platelet surface, in both sulfatide- and integrin receptor-bound states, and their balance controls the extent of the clotting response. The second major area of research targets to establish the role of the Tollip protein in innate immune responses and protein trafficking. Tollip modulates the Toll-like receptor (TLR) signaling pathway, which is required to promote innate immunity-related gene expression. Tollip is modular in nature with a TBD, C2, and CUE domains. We have recently established that the Tollip C2 domain exhibits broad preference for phosphoinositides, whereas the helical Tollip CUE domain binds monoubiquitin. Our interest is to understand how these domains act coordinately to modulate TLR signaling and membrane trafficking.