ABC TRANSPORTERS - BACTERIAL EXPORTERS

The ABC transport (also called traffic ATPases) make up a large superf amily of proteins which share a common function and a common ATP-bindi ng domain. ABC transporters are classified into three major groups: ba cterial importers (the periplasmic permeases), eukaryotic transporters , and bacterial exporters. We present a comprehensive review of the ba cterial ABC exporter group, which currently includes over 40 systems. The bacterial ABC exporter system are functionally subdivided on the b asis of the type of substrate that each translocates. We describe thre e main groups: protein exporters, peptide exporters, and systems that transport nonprotein substrates. Prototype exporters from each group a re described in detail to illustrate our current understanding of this protein family. The prototype systems include the alpha-hemolysin, co licin V, and capsular polysaccharide exporters from Escherichia coli, the protease exporter from Erwinia chrysanthemi, and the glucan export ers from Agrobacterium tumefaciens and Rhizobium meliloti. Phylogeneti c analysis of the ATP-binding domains from 29 bacterial ABC exporters indicates that the bacterial ABC exporters can be divided into two pri mary branches. One branch contains the transport systems where the ATP -binding domain and the membrane-spanning domain are present on the sa me polypeptide, and the other branch contains the systems where these domains are found on separate polypeptides. Differences in substrate s pecificity do not correlate with evolutionary relatedness. A complete survey of the known and putative bacterial ABC exporters is included a t the end of the review.