The enigmatic heptalaminate surface membrane of intravascular schistosomes

Intravascular schistosomes have an unusual outer, heptalaminate (seven-layered) covering consisting of not one but two lipid bilayers. Here, we present an updated model of the molecular composition of these bilayers in Schistosoma mansoni that places most identified proteins in the outer, and not the inner, membrane. Here, enzymes would have access to their recently described (non-membrane-permeable) substrates. By contrast, nutrient transporter proteins must be in both membranes to facilitate uptake into the worm's inner tissues. Ectoenzyme activities displayed by living worms suggest the presence on their outer surface of several noncanonically extracellular proteins. The advantages of having a double-bilayered covering may relate to impeding host immunological attack and/or to the worm's ability to acquire selected host molecules onto their exterior.