Canonical and ECF-type ATP-binding cassette importers in prokaryotes: diversity in modular organization and cellular functions

Since their discovery in the 1960s as 'osmotic shock-sensitive' transporters, a plethora of so-called binding protein-dependent (canonical) ATP-binding cassette (ABC) importers has been identified in bacteria and archaea. Their cellular functions go far beyond the uptake of nutrients. Canonical ABC importers play important roles in the maintenance of cell integrity, responses to environmental stresses, cell-to-cell communication and cell differentiation and in pathogenicity. A new class of abundant micronutrient importers, the 'energy-coupling factor' (ECF) transporters, was originally identified by functional genomics. ABC ATPases are an integral part of both canonical ABC and ECF importers. Fundamental differences include the modular architecture and the independence of ECF systems of extracytoplasmic solute-binding proteins. This review describes the roles of both types of transporters in diverse physiological processes including pathogenesis, points to the differences in modular assembly and depicts their common traits.