Thus, functionally characterized members of families 1, 5, and 7 are almost without exception specific for sugars; characterized members of families 2 and 3 are without exception specific for drugs and other deleterious substances; and families 4, 6, 8, 9, 11 to 14, and 17 are specific for various classes of anionic compounds. Furthermore, the only nucleoside permeases in the MFS are found in family 10, and most of the aromatic acid permeases are found in family 15. These observations clearly show that substrate specificity is a well-conserved trait and that phylogenetic classification provides a limited but reliable guide to function.
Similar considerations can be applied to pump polarity. Thus, while members of families 1, 5, 7, and 8 can apparently function quite readily by one or more modes (e.g., uniport, symport with inwardly directed polarity, and/or antiport), families 2, 3, 4, and 11 apparently function with a high propensity for an antiport mechanism, and families 6, 9, 10, and 12 to 15 probably function with a high propensity for a cation symport mechanism. Clearly, these mechanistic differences must reflect structural and catalytic residue differences, regardless of whether they reflect qualitative or quantitative differences. The molecular bases for these differences should be subject to biochemical, biophysical, and molecular genetic analyses.
