Protection of actin against proteolysis by complex formation with deoxyribonuclease I

G-actin bound to deoxyribonuclease I (DNase I) is resistant to digestion by trypsin and chymotrypsin. In the absence of DNase I, G-actin is cleaved by these proteases to yield a 33 500 molecular weight core protein which is not degraded further. The major sites of proteolytic action in the amino acid sequence of actin have been identified as being adjacent to residues arginine-62 and lysine-68 for trypsin and leucine-57 for chymotrypsin. These residues are rendered inaccessible to proteases in the buffer by complex formation with DNase I. Digestion of G-actin with pronase from Streptomyces griseus yields fragmentation patterns that are similar to those observed with trypsin and chymotrypsin. This is likely to be because the specificities of the major constituents of pronase resemble those of trypsin and chymotrypsin. Again, complex formation with DNase I protects the otherwise vulnerable bonds in actin against proteolysis. Incubation with subtilisin Carlsberg leads to complete digestion of G-actin. No subtilisin-resistant core protein accumulates during the incubation. Protection of G-actin when complexed to DNase I is less than complete in this case but still is significant. This is interpreted in terms of the broad specificity of subtilisin and the observed fragmentation pattern of free G-actin when treated with subtilisin.