Analysis of protein--protein relationships in 30S ribosome assembly intermediates using protection from proteolytic...

Treatment of the intact bacterial ribosome with proteolytic enzymes results in little or no digestion of many of the component proteins [Craven, G. R., & Gupta, V. (1970) Proc. Natl. Acad. Sci. U.S.A. 67, 1329]. In contrast, when the proteins are released from the constraints of ribosome structure they become completely susceptible to proteolytic attack. We have attempted to exploit these observations in an effort to determine the precise steps in ribosome assembly which result in a conversion of the structures of the various proteins from a proteolysis sensitive to a resistant form. Thus, a total of 11 30S ribosome assembly intermediate complexes of proteins and 16S RNA were prepared and digested with trypsin or chymotrypsin. The kinetics of digestion of each protein in the complex were followed by polyacrylamide gel electrophoresis. By a comparison of the digestion pattern of two complexes differing only by the presence of a single protein, it was possible to deduce several specific protective effects of one protein on its neighbor in the complex. On the basis of these studies, we propose nine protein-protein protective effects. The possible relevance of these interrelationships to other well-established proximity relationships is discussed.