SAPA

Saposin/surfactant protein-B A-type DOMAIN
SAPA
SMART accession number:SM00162
Description: Present as four and three degenerate copies, respectively, in prosaposin and surfactant protein B. Single copies in acid sphingomyelinase, NK-lysin amoebapores and granulysin. Putative phospholipid membrane binding domains.
Interpro abstract (IPR003119):

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 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. The Saposin-B domains also occur in other proteins, many of them active in the lysis of membranes [(PUBMED:8003971), (PUBMED:8868085)]. 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 [(PUBMED:8702672)].

Family alignment:
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There are 180 SAPA domains in 121 proteins in SMART's nrdb database.

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