Catalyzes the first step of diphthamide biosynthesis, a post-translational modification of histidine which occurs in elongation factor 2 (PubMed:30877278). DPH1 and DPH2 transfer a 3-amino-3-carboxypropyl (ACP) group from S-adenosyl-L-methionine (SAM) to a histidine residue, the reaction is assisted by a reduction system comprising DPH3 and a NADH-dependent reductase (By similarity). Acts as a tumor suppressor (PubMed:10519411).
