Academic Background

Arthur F.and Helen P. Koskinas Professor of Biochemistry
and Molecular Pharmacology

Solving the Protein Folding Problem

Determining the mechanism by which the amino acid sequence of a protein directs the rapid and efficient folding of proteins to their native, functional forms remains one of the most challenging problems in molecular biophysics. The development of a folding code that specifies the three-dimensional structure adopted by a given sequence would complement the genetic code and complete the central dogma of molecular biology (DNA " RNA " Amino Acid Sequence" Functional Protein). Over the past decade, it has also become clear that failure to fold correctly and quickly can lead to numerous diseases. The goals of our lab are to obtain a molecular-level understanding of the folding mechanisms of several very common motifs and to use those insights to contribute to the development of a general theory of protein folding. More recently, we have become interested in applying these biophysical insights to the development of therapeutics for misfolding diseases.

Ongoing projects in the lab

(βα)₈-barrel proteins

Tryptophan aporepressor (TR) (Figure 2) is a highly helical dimeric DNA-binding protein. The folding kinetics of TR are much more complex, involving three parallel pathways, with multiple steps along each pathway. A dimeric fragment corresponding to the first three helices that contains the hydrophobic core and the dimerization domain of the protein has been constructed and characterized. The folding kinetics of the TR core are much simpler than the full-length protein, permitting a direct measurement of the second-order rate constant for dimerization. We have currently extended our studies of oligomeric proteins to the folding of the trimeric, carbonic anhydrase-g, which contains a novel b-helix structure.

CheY –like proteins

Another class ofβα-repeat proteins is the CheY-like protein family, consisting of small (~ 125 residues) globular proteins that function as response regulators in bacterial two-component signal transduction pathways. Their five repeating βα units are arranged in an α/β/α sandwich topology: a central 5‑stranded β-sheet with helices 2, 3 and 4 on one surface of the sheet and helices 1 and 5 on the opposing surface. Several members of this class, including CheY, NT-NtrC and Spo0F (shown on the left), have been shown to fold via intermediates that are kinetically off the productive folding pathway. While the overall mechanism of folding for these proteins is similar and modulated by their common topology, we hypothesize that the structures of the folding intermediates and the details of the folding free-energy landscape are modulated by the amino acid sequence of the individual proteins. The sequence-local contacts between α-helices 2, 3 and 4 and the central β-sheet, define a large cluster of isoleucine, leucine and valine side chains in this early intermediate and provide a mechanism for the microsecond collapse to a kinetically-trapped species during refolding.

Protein folding and ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the loss of motor neurons in the brain and spinal cord. Generally ALS occurs later in life – the age of onset is generally between 40-60 years. After diagnosis, degeneration is rapid and the majority of patients die within 3 years. The largest subset of familial ALS (fALS) cases has been linked to mutations at the Cu, Zn superoxide dismutase (SOD1) locus. To date, over 150 mutations have been discovered, covering nearly 50% of the sequence and nearly all elements of secondary structure. ALS-variants of SOD1 often lead to the formation of insoluble aggregates in the motor neurons of afflicted individuals. Nevertheless, the conformations of SOD1 responsible for aggregation are unclear and a common molecular explanation for the formation of these aggregates has not been discovered. Recognizing that replacements of amino acids generally destabilize the protein, our lab seeks to quantitatively resolve the differences in the structure and folding properties of ALS-variants of SOD1 in an effort to determine a working model for aggregation and potential toxicity. It is also our intention to use insights into the folding mechanism of SOD1 to design assays for small molecular inhibitors of aggregation.

Techniques

Our lab utilizes a combination of spectroscopy and molecular biology to probe the folding properties of our various model systems. Rather than seek model systems amenable to a specific technique, our approach uses a broad array of techniques to obtain the maximum amount of information, as well as to ensure the accuracy and general applicability of the results. Furthermore, not all techniques are applicable to all proteins, due to limitations of size, solubility and stability. The broad expertise available in the lab allows for the study of systems otherwise intractable with an individual technique.

Probes of global structure, such as circular dichroism and small angle x-ray scattering, are utilized to measure the extent of collapse, and the shape of various folded and unfolded states. On the other hand, fluorescence spectroscopy, both intensity and lifetime measurements, are used to probe the local structure of tryptophan or other extrinsic fluorescent molecules. In combination, these methods are employed to measure the stability of the fold, as well as to ascertain whether particular regions of the protein are participating in folding intermediates. More detailed information on local and global collapse reactions are obtained by utilizing Förster resonance energy transfer (FRET) techniques. These methods, which leverage the exquisite distance sensitivity of field interactions between two fluorescent molecules, allow the measurement of distances between two positions in a protein molecule during refolding and unfolding.

To probe the extent of secondary structure formation in the native and intermediate states along a folding pathway we employ hydrogen exchange techniques coupled with either NMR or mass spectrometry. These methods exploit the fact that amide protons engaged in secondary structure formation, either by the formation of hydrogen bonds or by burial in the core of the protein, are less accessible to exchange with protons from the solvent. These techniques also hold the intriguing possibility of determining the regions of SOD1 which are involved in aggregation, by measuring the differences in protection patterns observed for the soluble protein compared to the aggregated protein.