The gene sequence of Manduca sexta juvenile hormone diol kinase (JHDK) codes for an enzyme that has 59% sequence identity to Drosophila melanogaster sarcoplasmic calcium-binding protein-2 (dSCP2). JHDK and dSCP2 are similar to G-proteins with three conserved sequence elements involved in purine nucleotide binding. Both proteins contain two pairs of EF-hand motifs. Characterization and partial purification of the D. melanogaster homolog of M. sexta JHDK from adult D. melanogaster gave material with JHDK activity. This activity has an experimental pI and molecular mass that are nearly identical to those of dSCP2. Moreover, D. melanogaster phosphotransferase activity has very similar chromatographic retention in three systems compared with M. sexta JHDK. Substrate docking to three-dimensional models of JHDK has shown that the three conserved nucleotide-binding elements surround the putative substrate-binding site and align with conserved sequence elements of p21(Ras) and adenylate kinase. D. melanogaster dSCP2 is a homolog of M. sexta JHDK, and these proteins constitute a novel kinase family that binds nucleotides using the scaffold of an SCP (Protein Data Bank code ).