View source for 1m12


==NMR solution structure of human Saposin C==
<StructureSection load='1m12' size='340' side='right'caption='[[1m12]]' scene=''>
== Structural highlights ==
<table><tr><td colspan='2'>[[1m12]] is a 1 chain structure with sequence from [https://en.wikipedia.org/wiki/Homo_sapiens Homo sapiens]. Full experimental information is available from [http://oca.weizmann.ac.il/oca-bin/ocashort?id=1M12 OCA]. For a <b>guided tour on the structure components</b> use [https://proteopedia.org/fgij/fg.htm?mol=1M12 FirstGlance]. <br>
</td></tr><tr id='method'><td class="sblockLbl"><b>[[Empirical_models|Method:]]</b></td><td class="sblockDat" id="methodDat">Solution NMR, 20 models</td></tr>
<tr id='resources'><td class="sblockLbl"><b>Resources:</b></td><td class="sblockDat"><span class='plainlinks'>[https://proteopedia.org/fgij/fg.htm?mol=1m12 FirstGlance], [http://oca.weizmann.ac.il/oca-bin/ocaids?id=1m12 OCA], [https://pdbe.org/1m12 PDBe], [https://www.rcsb.org/pdb/explore.do?structureId=1m12 RCSB], [https://www.ebi.ac.uk/pdbsum/1m12 PDBsum], [https://prosat.h-its.org/prosat/prosatexe?pdbcode=1m12 ProSAT]</span></td></tr>
</table>
== Disease ==
[https://www.uniprot.org/uniprot/SAP_HUMAN SAP_HUMAN] Defects in PSAP are the cause of combined saposin deficiency (CSAPD) [MIM:[https://omim.org/entry/611721 611721]; also known as prosaposin deficiency. CSAPD is due to absence of all saposins, leading to a fatal storage disorder with hepatosplenomegaly and severe neurological involvement.<ref>PMID:1371116</ref> <ref>PMID:11309366</ref>   Defects in PSAP saposin-B region are the cause of leukodystrophy metachromatic due to saposin-B deficiency (MLD-SAPB) [MIM:[https://omim.org/entry/249900 249900]. MLD-SAPB is an atypical form of metachromatic leukodystrophy. It is characterized by tissue accumulation of cerebroside-3-sulfate, demyelination, periventricular white matter abnormalities, peripheral neuropathy. Additional neurological features include dysarthria, ataxic gait, psychomotr regression, seizures, cognitive decline and spastic quadriparesis.  Defects in PSAP saposin-C region are the cause of atypical Gaucher disease (AGD) [MIM:[https://omim.org/entry/610539 610539]. Affected individuals have marked glucosylceramide accumulation in the spleen without having a deficiency of glucosylceramide-beta glucosidase characteristic of classic Gaucher disease, a lysosomal storage disorder.<ref>PMID:2060627</ref> <ref>PMID:17919309</ref>   Defects in PSAP saposin-A region are the cause of atypical Krabbe disease (AKRD) [MIM:[https://omim.org/entry/611722 611722]. AKRD is a disorder of galactosylceramide metabolism. AKRD features include progressive encephalopathy and abnormal myelination in the cerebral white matter resembling Krabbe disease.<ref>PMID:15773042</ref>   Note=Defects in PSAP saposin-D region are found in a variant of Tay-Sachs disease (GM2-gangliosidosis).
== Function ==
[https://www.uniprot.org/uniprot/SAP_HUMAN SAP_HUMAN] The lysosomal degradation of sphingolipids takes place by the sequential action of specific hydrolases. Some of these enzymes require specific low-molecular mass, non-enzymic proteins: the sphingolipids activator proteins (coproteins).  Saposin-A and saposin-C stimulate the hydrolysis of glucosylceramide by beta-glucosylceramidase (EC 3.2.1.45) and galactosylceramide by beta-galactosylceramidase (EC 3.2.1.46). Saposin-C apparently acts by combining with the enzyme and acidic lipid to form an activated complex, rather than by solubilizing the substrate.  Saposin-B stimulates the hydrolysis of galacto-cerebroside sulfate by arylsulfatase A (EC 3.1.6.8), GM1 gangliosides by beta-galactosidase (EC 3.2.1.23) and globotriaosylceramide by alpha-galactosidase A (EC 3.2.1.22). Saposin-B forms a solubilizing complex with the substrates of the sphingolipid hydrolases.  Saposin-D is a specific sphingomyelin phosphodiesterase activator (EC 3.1.4.12).
== Evolutionary Conservation ==
[[Image:Consurf_key_small.gif|200px|right]]
Check<jmol>
  <jmolCheckbox>
    <scriptWhenChecked>; select protein; define ~consurf_to_do selected; consurf_initial_scene = true; script "/wiki/ConSurf/m1/1m12_consurf.spt"</scriptWhenChecked>
    <scriptWhenUnchecked>script /wiki/extensions/Proteopedia/spt/initialview03.spt</scriptWhenUnchecked>
    <text>to colour the structure by Evolutionary Conservation</text>
  </jmolCheckbox>
</jmol>, as determined by [http://consurfdb.tau.ac.il/ ConSurfDB]. You may read the [[Conservation%2C_Evolutionary|explanation]] of the method and the full data available from [http://bental.tau.ac.il/new_ConSurfDB/main_output.php?pdb_ID=1m12 ConSurf].
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== Publication Abstract from PubMed ==
Saposin C binds to membranes to activate lipid degradation in lysosomes. To get insights into saposin C's function, we have determined its three-dimensional structure by NMR and investigated its interaction with phospholipid vesicles. Saposin C adopts the saposin-fold common to other members of the family. In contrast, the electrostatic surface revealed by the NMR structure is remarkably different. We suggest that charge distribution in the protein surface can modulate membrane interaction leading to the functional diversity of this family. We find that the binding of saposin C to phospholipid vesicles is a pH-controlled reversible process. The pH dependence of this interaction is sigmoidal, with an apparent pK(a) for binding close to 5.3. The pK(a) values of many solvent-exposed Glu residues are anomalously high and close to the binding pK(a). Our NMR data are consistent with the absence of a conformational change prior to membrane binding. All this information suggests that the negatively charged electrostatic surface of saposin C needs to be partially neutralized to trigger membrane binding. We have studied the membrane-binding behavior of a mutant of saposin C designed to decrease the negative charge of the electrostatic surface. The results support our conclusion on the importance of protein surface neutralization in binding. Since saposin C is a lysosomal protein and pH gradients occur in lysosomes, we propose that lipid degradation in the lysosome could be switched on and off by saposin C's reversible binding to membranes.

Solution structure of human saposin C: pH-dependent interaction with phospholipid vesicles.,de Alba E, Weiler S, Tjandra N Biochemistry. 2003 Dec 23;42(50):14729-40. PMID:14674747<ref>PMID:14674747</ref>

From MEDLINE&reg;/PubMed&reg;, a database of the U.S. National Library of Medicine.<br>
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<div class="pdbe-citations 1m12" style="background-color:#fffaf0;"></div>

==See Also==
*[[Saposin|Saposin]]
== References ==
<references/>
__TOC__
</StructureSection>
[[Category: Homo sapiens]]
[[Category: Large Structures]]
[[Category: Tjandra N]]
[[Category: Weiler S]]
[[Category: De Alba E]]