Structure and function of haemolysin B,P-glycoprotein and other members of a novel family of membrane translocators.

Recent studies have identified two sub-families of highly conserved polypeptides in a wide variety of organisms concerned with the transport of many different compounds, specific for each transport protein. Both families, represented by HisP and HlyB, respectively, have in common a highly conserved, approximately 25 kD domain, containing an ATP-binding site. The HisP sub-family essentially consists of cytoplasmic proteins which couple energy to the import of small substrates through cytoplasmic membrane permeases in Gram-negative bacteria. The HlyB (P-glycoprotein) sub-family, on the other hand, contains a second large domain which apparently acts as the transmembrane translocator itself, which in most cases drives the secretion of a variety of compounds. These membrane domains share a number of structural features which also serve to distinguish these proteins as a closely related group. Nevertheless, the compounds secreted by the HlyB sub-family include large polypeptides, polysaccharides and a variety of anti-tumour drugs. We describe here the properties of each of these remarkable proteins and we speculate on their possible mechanism of action.