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

Summary 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 famines, represented by HisP and HlyB, respectively, have in common a highly conserved, approximately 25kD 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.