A Yeast Expression System of High Efficiency for Producing Recombinant Enzymes

Notwithstanding their structural and functional differences, cADPR and NAADP are synthesized by the same homologous enzymes. Three have so far been identified and they are the Aplysia ADP-ribosyl cyclase and the mammalian antigens, CD38 and CD157 (formerly called BST-1) [1–4]. The sequence homology among these three enzymes is shown in Figure 1. All three enzymes are widespread in occurrence and are localized in a variety of tissues [1, 5–8]. The novel multifunctionality of these enzymes calls for detailed structural studies of their catalytic properties by site-directed mutagenesis, enzyme kinetics and X-ray crystallography. Large quantities of proteins are needed for these kinds of studies, necessitating the development of an efficient expression system for producing the necessary recombinant proteins. In addition, the Aplysia cyclase is the most critical component of the recently developed cycling assay for cADPR [9]. CD38, on the other hand, is the only known enzyme that can hydrolyze cADPR and is used in its radioimmunoassay to verify that the signal is indeed due to cADPR [10].