Deletion of amino acids from the carboxy-terminal end of actin

A series of deletions was made from the C-terminal end of actin by inserting termination codons into a full length cDNA of human alpha-skeletal muscle actin. These included deletions of 2, 3, 10, 20, 30, and 40 amino acids. The cDNA clones were transcribed and the resulting mRNAs were translated in vitro using 35S-labeled methionine. The 35S-labeled actin and actin mutants were then tested for the ability to coassemble with carrier actin, bind DNAse I, bind myosin S-1, bind a 27 kDa proteolytic fragment of alpha-actinin, and incorporate into myofibrils in vitro. Removal of the C-terminal two or three amino acids did not grossly alter the properties of actin tested. Deletion of an additional 7 amino acids (10 amino acids total) significantly decreased coassembly, binding to DNAse I, and incorporation into myofibrils, but did not dramatically reduce binding to myosin S-1 or the 27 kDa fragment of alpha-actinin. Deletion of 20 or more amino acids virtually abolished all normal actin function tested. By examining the structure of actin, we propose that the effect of removing residues 356-365 is due to the important role Trp356 plays in maintaining hydrophobic bonds between three non-contiguous segments of actin. We also suggest that removal of residues 366-372 adversely affected the structure or orientation of the DNAse I binding loop and that this change can account for defects in actin binding to DNAse I, coassembly with wild type actin, and incorporation into myofibrils.