A NEW CLASS OF PHOSPHOTRANSFERASES PHOSPHORYLATED ON AN ASPARTATE RESIDUE IN AN AMINO-TERMINAL DXDX(T V) MOTIF/

When incubated with their substrates, human phosphomannomutase and L-3 -phosphoserine phosphatase are known to form phosphoenzymes with chemi cal characteristics of an acyl-phosphate, The phosphorylated residue i n phosphomannomutase has now been identified by mass spectrometry afte r reduction of the phosphoenzyme with tritiated borohydride and trypsi n digestion. It is the first aspartate in a conserved DVDGT motif. Rep lacement of either aspartate of this motif by asparagine or glutamate resulted in complete inactivation of the enzyme. The same mutations pe rformed in the DXDST motif of L-3-phosphoserine phosphatase also resul ted in complete inactivation of the enzyme, except for the replacement of the second aspartate by glutamate, which reduced the activity by o nly about 40%, This suggests that the first aspartate of the motif is also the phosphorylated residue in L-3-phosphoserine phosphatase. Data banks contained seven other phosphomutases or phosphatases sharing a similar, totally conserved DXDX(TN) motif at their amino terminus. One of these (beta-phosphoglucomutase) is shown to form a phosphoenzyme w ith the characteristics of an acyl-phosphate. In conclusion, phosphoma nnomutase and L-3-phosphoserine phosphatase belong to a new phosphotra nsferase family with an amino-terminal DXDX(TN) motif that serves as a n intermediate phosphoryl acceptor.