Aminoacyl carrier protein. Can be charged with L-alanine, L-glycine or L-serine, via the formation of a thioester bond between the amino acid and the 4'-phosphopantetheinyl prosthetic group, catalyzed by the Atu2573 ligase. 4'-phosphopantetheine is transferred from CoA to a specific serine of the apo-form of this carrier protein.