Saposins are small lysosomal proteins that serve as activators of variouslysosomal lipid-degrading enzymes [<cite idref="PUB00005747"/>]. They probably act by isolating thelipid substrate from the membrane surroundings, thus making it more accessible to the soluble degradative enzymes. All mammalian saposinsare synthesised as a single precursor molecule (prosaposin) which containsfour Saposin B domains, yielding the active saposins after proteolyticcleavage, and two Saposin-A domains that are removed in the activationreaction. The Saposin B domains also occur in other proteins, many of them active in the lysis of membranes [<cite idref="PUB00005721"/>, <cite idref="PUB00005765"/>]. <p>The 3D-structure of NK-lysin has recently been determined [<cite idref="PUB00005798"/>] and found tobe very different from the one predicted in [<cite idref="PUB00005747"/>].A group of plant aspartic proteases related to cyprosin, which have a peculiar SAP-B domain where the two halves are 'swapped' [<cite idref="PUB00005742"/>].</p>