<p>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"/>]. The saposin A-type domain may play a role in targeting, as propeptides containing the saposin A-type domain of the C terminus of prosaposin and of the N-terminal part of pulmonary surfactant-associated protein B are involved in the transport to the lysosome and to secretory granules (lamellar bodies, which are lysosomal-like organelles), respectively [<cite idref="PUB00016998"/>].</p>