The goal of this proposal is to elucidate the conformation of N-terminal polypeptides of increasing length (belonging to the sequence of an all-alpha-helical single domain model protein) in the presence of the cotranslationally active chaperone Hsp70. In order to obtain information at amino-acid specific resolution, multidimensional NMR in the presence of isotopically labeled peptide and unlabeled chaperone will be employed. Other biophysical methods such as isothermal titration calorimetry, size exclusion chromatography and fluorescence spectroscopy will also be used. The work focuses on elongating N-terminal polypeptides derived from apomyoglobin, a relatively small and extremely well characterized system which serves as an excellent model for all-alpha-helical proteins. The proposed investigations will explore whether Hsp70 merely prevents interchain aggregation by holding a statistical coil status, or it also acts by inducing specific polypeptide conformations. This study is not intended to directly mimic intracellular cotranslational and immediately posttranslational folding events. On the other hand, it aims at providing a first order in vitro approximation to how Hsp70 is able to affect the conformational space of elongating polypeptides. The expected influence of specific cell-related effects such as polypeptide tethering (to the ribosome exit channel) and molecular crowding are discussed in the proposal and will be addressed by separate additional experiments. Very little is known about the mechanisms by which Hsp70, the main cotranslationally active chaperone, influences the course of protein folding. Yet, progress is urgently needed in this area since defective action (or insufficient bioavailable amount) of cotranslational chaperones has been linked to the formation of incorrectly folded self-associated species such as those involved in a number of deadly diseases. These include cystic fibrosis, inflammatory heart disease, Crohn disease, P53-related cancers, and several neurodegenerative disorders such as Huntington's and Alzheimer's disease. Experiments to be carried out include (a) high resolution secondary structure mapping of isotopically labeled polypeptides by NMR in the presence of unlabeled Hsp70 chaperone; (b) hydrogen/deuterium exchange pulse labeling kinetic experiments to detect the mechanisms of structure formation in the presence of Hsp70; (c) additional studies in the presence of the Hsp40 and Hsp70-nucleotide exchange factor cochaperones.
