Table 1. Number of human homologues for the Escherichia coli and Saccharomyces cerevisiae polyamine transporters.

The amino acid sequences were retrieved from the NCBI protein database or the SGD database for polyamine transporters of Escherichia coli and Saccharomyces cerevisiae and subjected to a standard blast search in NCBI (algorithm blastp). The chosen query database was “RefSeq Protein”. The search was restricted to Homo sapiens. For CadB (E. coli) two possible sequences were retrieved (147 and 444 amino acids) and both were used in our search. The shorter protein is part of the longer one. For MdtII no protein but rather a chain A and a chain B of the Mdt protein were found. Both chains were used in our search. For all transporters, only the homologues with an E value below 10^-4 were included. Applying these parameters, the list of proteins yielded in this search was big. The results can be subdivided into 3 groups: (i) those proteins without any apparent homolog; (ii) proteins with 1-6 homologs and (iii) proteins with a long list of homologs. While the result for group (i) is clear, the results for group (ii) are all at the edge of the applied threshold and none of them is an overwhelming hit. As for group (iii) the long list might be due to the homology of a conserved functional domain and to the inclusion of all isoforms of a specific hit (see e.g. homologues for PotA in E. coli). The full list of homologues can be found in the Supplementary Table.

Transporters	Number of human homologues
Saccharomyces cerevisiae
QDR3, TPO4, TPO5	1
TPO2, SAM3	2
TPO3	3
AGP2, DUR3	5
TPO1, GAP1	6
UGT4	19
Escherichia coli
PotB, PotC, PotH, PotI	0
PotD, PotF	1
CadB	10
PotE	12
PotA, PotG	75+