Aminocarboxypropyltransferase that catalyzes the aminocarboxypropyl transfer on pseudouridine at position 1248 (Psi1248) in 18S rRNA (Probable). It constitutes the last step in biosynthesis of the hypermodified N1-methyl-N3-(3-amino-3-carboxypropyl) pseudouridine (m1acp3-Psi) conserved in eukaryotic 18S rRNA (Probable). . This is the function of ENSG00000007520 (TSR3, TSR3 ribosome maturation factor).