Arthur L. Horwich

Professor of Genetics and Pediatrics; Associate Investigator, Howard Hughes Medical Institute

A.B. Brown University 1972
M.D. Brown University, 1975

Research Interests:

Chaperones in protein folding and unfolding
Protein misfolding/amyloid formation

Honors:

Basil O'Connor Research Award
John A. Hartford Foundation Fellowship

The laboratory has been pursuing studies in the area of protein folding in vivo for some years, now focusing on three particular systems. One is the chaperonin ring system, that provides ATP-driven assistance in folding proteins to their native state in many cellular compartments. A second is the Clp proteolytic system, which also uses ATP and a ring structure, but to unfold proteins and then degrade them inside a proteasome like cylinder. A third area of study involves the mechanism of protein misfolding resulting in the formation of amyloids, characteristic fibrillar structures, involved in a variety of neurodegenerative diseases.

Current Research

The chaperonin studies focus on the mechanism of action of the bacterial chaperonin, GroEL, and its cochaperonin, GroES, in mediating protein folding. Our structural and mechanistic studies of the past few years have identified some of the basic workings: a non-native protein is bound inside an open GroEL ring through hydrophobic contacts between exposed surfaces in the non-native protein and hydrophobic side chains of the apical lining of the ring; ATP and GroES become bound to the same GroEL ring, causing large movements of the apical domains, which twists them away from the bound polypeptide, releasing it into the GroEL cavity, now encapsulated by the bound "lid"-like cochaperonin, where it commences folding in isolation. Following timed hydrolysis of ATP in the folding-active ring and ATP binding in the opposite GroEL ring, the bound GroES and the folding polypeptide are ejected into solution. If the protein has reached native form, it proceeds to carry out its function in the cell; if it has not reached native form, it can be rebound and try once again to reach the native state. While we understand some of the workings of the GroEL machine and recognize that productive folding occurs in its cavity, we have little resolution of what exactly happens to the polypeptide substrate. Our current studies focus on "seeing" what a polypeptide looks like while bound to GroEL, on testing whether binding of ATP/GroES causes it to be "stretched on the rack" prior to its complete release into the cavity, as has been suggested by others, and on whether binding to GroEL is itself associated with unfolding. A variety of biochemical and biophysical approaches are being taken, including crystallographic and spectroscopic.

A second ring system involving the ClpA chaperone, also an ATP utilizing ring, and its cognate protease, ClpP, which resembles the proteasome insofar as containing a narrow central channel into a cavity filled with proteolytic "teeth," seems to carry out an action opposite that of GroEL, ripping apart the structures of native proteins bearing covalent peptide tags and feeding/translocating the unfolded substrates into the coaxially aligned proteolytic component for degradation. Here we seek to understand how ClpA is able to function as an "unfoldase." We are currently studying the translocation of unfolded proteins, examining the directionality of "feeding" into the protease using fluorescence dynamics approaches. We are also seeking to examine the mechanics of the unfolding reaction, following up on our observations that the substrate becomes globally unfolded, with kinetic studies aimed at observing whether this is also directional. We are also seeking to produce well-diffracting crystals of ClpA to determine its structure.

In the third area of study, we're interested to understand how amyloids are formed, beginning with a basic study of the structure, as yet unknown, of the end-product of such a reaction, an amyloid itself. We would also like to know whether molecular chaperones can influence this process in vivo. Since most amyloids are formed from secretory proteins, the question arises whether there are chaperones in the later parts of the secretory pathway where such "conversion" occurs, which can influence the formation of the toxic amyloidogenic folding intermediates and the end-product fibrils.

Representative Publications:

Horwich, A.L., Weber-Ban, E., and Finley, D. (1999). Chaperone rings in protein folding and degradation. Proc. Natl. Acad. Sci. USA 96, 11033-11040.

Rye, H.S., Roseman, A.M., Furtak, K., Fenton, W.A., Saibil, H.R., and Horwich, A.L. (1999). GroEL-GroES cycling: ATP and non-native polypeptide direct alternation of folding-active rings. Cell 97, 325-338.

Weber-Ban, E.U., Reid, B.G., Miranker, A.D., and Horwich, A.L. (1999). Global unfolding of a substrate protein by the Hsp100 chaperone ClpA. Nature 401, 90-93.

Farr, G., Furtak, K., Rowland, M., Ranson, N.A., Saibil, H.R., Kirchhausen, T., and Horwich, A.L. (2000). Multivalent binding of non-native substrate proteins by the chaperonin GroEL. Cell 100, 561-573.

Updated 1 September 2000