Protein anatomy: functional roles of barnase module.

Globular proteins are composed of several modules that are contiguous polypeptide segments of compact conformation. Module boundaries are closely correlated with the intron positions of genes that encode proteins. The modules may thus have a one-to-one correspondence with exons in primordial genes. They may also be vestiges of polypeptide segments that initially appeared as primordial proteins in prebiological evolution. Clarification as to whether modules disconnected from one another have functional potentiality may validate these possibilities. Thus, in this study, each module of a protein was synthesized and assessed for functional potentiality. For this purpose, barnase, a bacterial ribonuclease, was decomposed into six modules (M1-M6), which were examined to determine whether they have an affinity for RNA and RNase activity. M2, M3, and M6, all of which form a shallow but wide cavity for RNA binding in native barnase, were found to bind to RNA and to possess RNase activity. However, M1 and M5, which support the other modules from the back side, and M4 did not bind to RNA and had no RNase activity. Protein modules with catalytic functions are described in this paper for the first time. That some modules of barnase possess catalytic activity indicates that protein modules may possibly have functioned as primitive catalysts in prebiological evolution.