Saposin protein domain

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 (prosaposin) 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.

[2] The Saposin-B domains also occur in other proteins, most of them playing a role in interacting with membranes.

It also includes the N- and C-terminal SapA domains, both of which are proteolyticly cleaved as the proprotein matures.

Four connected pairs of SapB1-SapB2 domains are released, sequentially named Saposin-A through D. Some closely related proteins, such as PSAPL1 and SFTPB, share the architecture and the cleaving mechanism in whole or in part.