Our research is focused on proteins that self-assemble into hierarchical functional structures.  The main system we are currently working with is adiponectin

Adiponectin

Control of Adiponectin Activity by Oligomerization:

Adiponectin is a protein secreted exclusively by adipocytes.  It is emerging as a key player in regulating energy balance, and it’s misregulation is strongly tied to Type II diabetes.  Epidemiological studies have shown that diabetics, obese people have abnormally low levels of adiponectin.  Adiponectin knockout mice are strongly susceptible to diet-induced insulin resistance and administration of adiponectin increases insulin sensitivity.

Adiponectin is found in the body in several forms, as a trimer, hexamer and a high molecular weight species that is still poorly characterized.  Remarkably, the oligomeric state changes the tissue specificity and activity of the protein.  The trimer acts on muscle cells to stimulate fatty acid oxidation; the hexamer can suppress hepatic glucose output.  Furthermore, thiazolidinediones, which are used to treat type II diabetes, increase the ratio of high molecular weight complex relative to the other species.  Thus, it is important to obtain large quantities of the high molecular weight form and characterize its oligomeric state.  This information will then be correlated with its biological activity as we uncover it in collaboration with the Lodish Lab at the Whitehead Institute.

We are currently purifying all three forms from adipocytes.  We will use spectroscopic methods as well as analytical ultracentrifugation to characterize their oligomeric state and secondary structure.

Schematic representation of adiponectin domain structure and oligomeric state.  Adiponectin has an N-terminal collagen domain followed by a globular domain.  The trimer is thought to be the basic building block for higher oligomers.

Ribbon representation of the trimeric globular domain of adiponectin. The globular domain of adiponectin  has a 10-stranded stranded jelly roll topology that is structurally homologous to the TNF-a family members.  Interestingly, it self-assembles as a homotrimer without the collagen domain.  Each monomer is colored blue, green, or red.

Cryoelectron micrographs of the trimeric (left), hexameric (middle) and high molecular weight (right) forms of adiponectin.  Bar is 14.2 nm.  From our publication in J. Biol. Chem., 278, 50810-50817 (2003). 

Left. Equilibrium sedimentation trace of hexameric adiponectin from E. coli.  Right. . Equilibrium sedimentation trace of trimeric adiponectin from E. coli.  The random residuals for both curves indicate that the hexamer and trimer behave as ideal single species in solution and do not equilibrate with alternate oligomers. From our publications in J. Biol. Chem., 278, 50810-50817 (2003) and J. Biol. Chem. 277, 29359-29362 (2002).