Structural and membrane-binding properties of saposin D.

Area of Science:

• Biochemistry
• Cell Biology
• Structural Biology

Background:

• Saposin D is a protein derived from prosaposin in acidic organelles like lysosomes.
• Its role in sphingomyelin and ceramide hydrolysis is known, but its physiological function remains unclear.
• Understanding saposin D's interaction with membranes is crucial for elucidating its function.

Purpose of the Study:

• To investigate the structural and membrane-binding properties of saposin D.
• To determine the factors influencing saposin D's association with lipid bilayers.
• To explore the role of saposin D in the acidic environments of late endosomes and lysosomes.

Main Methods:

• Mass spectrometry (MS) analysis to determine structural properties, including amino acid sequence and disulfide bond linkage.
• Investigation of saposin D's binding affinity to phospholipid membranes with varying compositions at acidic pH.
• Analysis of the N-glycosylation site and its effect on membrane interaction.
• Monitoring changes in saposin D's intrinsic tyrosine fluorescence upon membrane association.

Main Results:

• Saposin D exhibits high affinity for membranes containing anionic phospholipids (phosphatidylserine, phosphatidic acid) at acidic pH.
• The protein's binding destabilizes the lipid surface and alters its own fluorescence properties.
• Saposin D has a specific disulfide bond pattern (Cys5-Cys78, Cys8-Cys72, Cys36-Cys47), common to other lipid-binding saposins.
• Deglycosylation does not affect saposin D's membrane interaction, indicating glycosylation is not essential for lipid binding.

Conclusions:

• Anionic phospholipids are key determinants of saposin D's localization and interaction within acidic organelles.
• Saposin D's structural features, particularly its disulfide bonds, are critical for its membrane-binding capabilities.
• Saposin D may play a role in organizing lipids within anionic phospholipid-rich membranes in lysosomes and late endosomes.