Saposin (B) Domains
SMART accession number:SM00741
Description: Present in multiple copies in prosaposin and in pulmonary surfactant-associated protein B. In plant aspartic proteinases, a saposin domain is circularly permuted. This causes the prediction algorithm to predict two such domains, where only one is truly present.
Interpro abstract (IPR008139): Saposins are small lysosomal proteins that serve as activators of various lysosomal lipid-degrading enzymes [(PUBMED:7595087)]. 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 synthesised 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. The Saposin B domains also occur in other proteins, many of them active in the lysis of membranes [(PUBMED:8003971), (PUBMED:8868085)].

The 3D-structure of NK-lysin has recently been determined [(PUBMED:9334742)] and found to be very different from the one predicted in [(PUBMED:7595087)]. A group of plant aspartic proteases related to cyprosin, which have a peculiar SAP-B domain where the two halves are 'swapped' [(PUBMED:7610480)].

Family alignment:
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There are 3257 SapB domains in 1672 proteins in SMART's nrdb database.

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