Upon a stimulus of light, histamine is released from Drosophila photoreceptor axonal endings. It is taken up into glia where Ebony converts it into ␤-alanyl-histamine (carcinine). Carcinine moves into photoreceptor cells and is there cleaved into ␤-alanine and histamine by Tan activity. Tan thus provides a key function in the recycling pathway of the neurotransmitter histamine. It is also involved in the process of cuticle formation. There, it cleaves ␤-alanyl-dopamine, a major component in cuticle sclerotization. Active Tan enzyme is generated by a selfprocessing proteolytic cleavage from a pre-protein at a conserved Gly-Cys sequence motif. We confirmed the dependence on the Gly-Cys motif by in vitro mutagenesis. Processing time delays the rise to full Tan activity up to 3 h behind its putative circadian RNA expression in head. To investigate its pleiotropic functions, we have expressed Tan as a His 6 fusion protein in Escherichia coli and have purified it to homogeneity. We found wild type and mutant His 6 -Tan protein co-migrating in size exclusion chromatography with a molecular weight compatible with homodimer formation. We conclude that dimer formation is preceding pre-protein processing. Drosophila tan 1 null mutant analysis revealed that amino acid Arg 217 is absolutely required for processing. Substitution of Met 256 in tan 5 , on the contrary, does not affect processing extensively but renders it prone to degradation. This also leads to a strong tan phenotype although His 6 -Tan 5 retains activity. Kinetic parameters of Tan reveal characteristic differences in K m and k cat values of carcinine and ␤-alanyl-dopamine cleavage, which conclusively illustrate the divergent tasks met by Tan.