We have determined the partial amino acid sequence of p33, an endogenous substrate protein for arginine-specific ADP-ribosyltransferase in chicken polymorphonuclear leukocytes (heterophils), and found that the sequence was completely identical with the regions of amino acid sequences deduced from mim-1 (named for myb-induced myeloid protein-1, which is expressed in chicken promyelocytes) cDNA [(1989) Cell, 59, 1115–1125], except for one amino acid difference (Tyr297→Ile). These results together with data on cellular and subcellular distributions of p33 in heterophils suggest that mim-1 may encode the precursor protein of p33.
