Protein anatomy: Structure and function of peptide fragments correspondingto the secondary structure units of barnase

Globular proteins can be decomposed into several modules or secondary struc ture units, It is useful to investigate the functions of such structural un its in order to understand the folding units of proteins. In our previous w ork, barnase was divided into six peptide fragments corresponding to module s, and some of them were shown to have RNA-binding and RNase activity [Yana gawa, et al. (1993) J. Biol. Chem. 268, 5861-5865], Barnase mutant proteins obtained by permutation of the structural units also had RNase activity [T suji, T. et al. (1999) J. Mel. Biol. 286, 1581-1596], Here we investigated the structure and function of peptide fragments corresponding to secondary structure units of barnase, The results of circular dichroism spectroscopy indicated that some of the peptide fragments form helical structures in aqu eous solutions containing over 30% 2,2,2-trifluoroethanol, and the S6 (94-1 10) peptide fragment is induced to form a beta -sheet structure in the pres ence of RNA. The S6 peptide fragment forms aggregate complexes with RNA. El ectron microscopic analysis showed that the aggregate complexes were compri sed of filaments. These results indicate that not only modules but also sec ondary structure units dissected from a globular protein have functional an d structure-forming capabilities.