Title

The dynamic folding of SecA: From rapid dimer formation to multiple native states

Date of Completion

January 2002

Keywords

Chemistry, Biochemistry

Degree

Ph.D.

Abstract

The amino acid sequence of a protein ultimately determines both the folding pathway and the final structure of a protein. In order to better understand the folding of large, multimeric proteins, I used SecA as a model protein. SecA is a homodimer comprising 901 amino acids per monomer, and it undergoes many conformational changes during its in vivo role in precursor translocation. From my studies I have determined that SecA folds through parallel channels with multiple dimeric folding intermediates. The dimerization of SecA occurs on the millisecond time scale and is diffusion limited. This rapid dimerization may be a way in which SecA and other large proteins avoid aggregation during folding. In addition, SecA has two native state ensembles as well as two denatured states. The two native states are formed in 300 ms and 10 s, indicating that a large protein can fold in an amazingly short time. The unfolding rates are slower, suggesting the dimer is the most stable state and likely the functional unit in vivo. We propose that the complex dynamics of SecA shown here are important for SecA function. The multiple intermediate conformations may be physiologically relevant, in that SecA has a conformationally extended ATP-bound form, which may be in a partially unfolded state. For a protein with many ligands and one that undergoes multiple conformational changes during its role in vivo, having native, intermediate and unfolded ensembles may be a way that SecA can perform and control all of its in vivo functions. ^