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The saposin domains refers to two evolutionally-conserved protein domains found in saposin and related proteins. Saposins are small lysosomal proteins that serve as activators of various lysosomal lipid-degrading enzymes. They probably act by isolating the lipid substrate from the membrane surroundings, thus making it more accessible to the soluble degradative enzymes. All mammalian saposins are synthesized as a single precursor molecule which contains four Saposin-B domains, yielding the active saposins after proteolytic cleavage, and two Saposin-A domains that are removed in the activation reaction.
The Saposin-B domains also occur in other proteins, most of them playing a role in interacting with membranes.
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