Primary structure determination and physicochemical characterization of DSP-3, a phosphatidylcholine binding glycoprotein of donkey seminal plasma.

The researchers in this study focused on understanding the properties of DSP-3, a protein from the seminal fluid of donkeys. They found that the protein contains 106 amino acids and discovered interesting similarities and differences in structure compared to other seminal proteins. Their work also indicated differences in how these proteins bind to cell membranes and demonstrated that the DSP-3 molecule changes its structure and improves thermal stability when it interacts with certain lipids.

Protein Analysis

• DSP-3, a major protein found in the seminal plasma of donkeys, was the focus of this study. It belongs to a family of proteins, referred to as the FnII protein family, that also include bovine PDC-109, equine HSP-1/2, and donkey DSP-1.
• The researchers used high-resolution mass-spectrometric studies to determine that the DSP-3 protein consists of 106 amino acid residues and further observed multiple acetylations on the glycans, suggesting it is heterogeneously glycosylated.

Protein Comparisons

• A comparative analysis found that DSP-3 shares more identical amino acid residues with HSP-1 (118 out of 106) than with DSP-1, another known seminal plasma protein from donkeys (only 72 out of 106).
• It was also discovered, using circular dichroism (CD) spectroscopic and differential scanning calorimetric (DSC) studies, that the DSP-3 nonetheless shows a difference in structure as compared to PDC-109 and DSP-1.

Thermal Stability and Ligand Binding

• DSP-3 unfolds at approximately 45 degrees Celsius. However, upon binding to phosphorylcholine (PrC), which is a component of certain lipids in the cell membrane, DSP-3’s thermal stability increases.
• This aspect was further scrutinised using ligand binding studies, where it was observed that DSP-3 binds lyso-phosphatidylcholine with an estimated 80-fold higher affinity than PrC.
• Unlike PDC-109 and DSP-1 that exist as mixtures of polydisperse oligomers, DSP-3 likely exists as a monomer based on DSC data analysis.

Physiological Implications

• When DSP-3 binds to erythrocytes, it causes changes to the membrane, implying a similar interaction could occur on the sperm plasma membrane. This suggests a potential physiological significance of DSP-3’s capability to bind and interact with cell membranes in the reproductive context.