The evolutionary repertoires of the eukaryotic-type ABC transporters in terms of the phylogeny of ATP-binding domains in eukaryotes and prokaryotes.

ABC (ATP-binding cassette) transporters play an important role in the communication of various substrates across cell membranes. They are ubiquitous in prokaryotes and eukaryotes, and eukaryotic types (EK-types) are distinguished from prokaryotic types (PK-types) in terms of their genes and domain organizations. The EK-types and PK-types mainly consist of exporters and importers, respectively. Prokaryotes have both the EK-types and the PK-types. The EK-types in prokaryotes are usually called "bacterial multidrug ABC transporters," but they are not well characterized in comparison with the multidrug ABC transporters in eukaryotes. Thus, an exhaustive search of the EK-types among diverse organisms and detailed sequence classification and analysis would elucidate the evolutionary history of EK-types. It would also help shed some light on the fundamental repertoires of the wide variety of substrates through which multidrug ABC transporters in eukaryotes communicate. In this work, we have identified the EK-type ABC transporters in 126 prokaryotes using the profiles of the ATP-binding domain (NBD) of the EK-type ABC transporters from 12 eukaryotes. As a result, 11 clusters were identified from 1,046 EK-types ABC transporters. In particular, two large novel clusters emerged, corresponding to the bacterial multidrug ABC transporters related to the ABCB and ABCC families in eukaryotes, respectively. In the genomic context, most of these genes are located alone or adjacent to genes from the same clusters. Additionally, to detect functional divergences in the NBDs, the Kullback-Leibler divergence was measured among these bacterial multidrug transporters. As a result, several putative functional regions were identified, some corresponding to the predicted secondary structures. We also analyzed a phylogeny of the EK-type ABC transporters in both prokaryotes and eukaryotes, which revealed that the EK-type ABC transporters in prokaryotes have certain repertoires corresponding to the conventional ABC protein groups in eukaryotes. On the basis of these findings, we propose an updated evolutionary hypothesis in which the EK-type ABC transporters in both eukaryotes and prokaryotes consisted of several kinds of ABC transporters in putative ancestor cells before the divergence of eukaryotic and prokaryotic cells.