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 synthesized 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"/>].