The Nucleotide Transporter of Caedibacter caryophilus Exhibits an Extended Substrate Spectrum Compared to the Analogous ATP/ADP Translocase of Rickettsia prowazekii

The two obligate intracellular alphaproteobacteria Rickettsia prowazekii and Caedibacter caryophilus , a human pathogen and a paramecium endosymbiont, respectively, possess transport systems to facilitate ATP uptake from the host cell cytosol. These transport proteins, which have 65% identity at the amino acid level, were heterologously expressed in Escherichia coli , and their properties were compared. The results presented here demonstrate that the caedibacter transporter had a broader substrate than the more selective rickettsial transporter. ATP analogs with modified sugar moieties, dATP and ddATP, inhibited the transport of ATP by the caedibacter transporter but not by the rickettsial transporter. Both transporters were specific for di- and trinucleotides with an adenine base in that adenosine tetraphosphate, AMP, UTP, CTP, and GTP were not competitive inhibitors. Furthermore, the antiporter nature of both transport systems was shown by the dependence of the efflux of [α- 32 P]ATP on the influx of substrate (ATP but not dATP for rickettsiae, ATP or dATP for caedibacter).