Super-channel in bacteria: function and structure of a macromolecule import system mediated by a pit-dependent ABC transporter.

In a soil isolate, Sphingomonas sp. A1, the transport of a macromolecule (alginate: 27 kDa) is mediated by a pit-dependent ATP-binding cassette (ABC) transporter. The transporter is different from other ABC transporters so far analyzed in that its function is dependent on the pit, a mouth-like organ formed on the cell surface only when the cells are compelled to assimilate macromolecules, and in that it allows direct import of macromolecules into cells. The ABC transporter coupled with the pit, which functions as a funnel and/or concentrator of macromolecules to be imported, was designated as the "Super-channel", and in this review, we discuss the three-dimensional structure and specific function of the "Super-channel" for macromolecule import found for the first time in a bacterium.