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Alternative proteolytic processing of platelet membrane glycoprotein IIb
J C Loftus1, E F Plow, L K Jennings
1Research Institute of Scripps Clinic, Department of Immunology, La Jolla, California 92037.
Abstract:
Platelet membrane glycoprotein (GP) IIb-IIIa is a component of a receptor for the adhesive proteins fibrinogen, fibronectin, and von Willebrand factor. GPIIb is initially synthesized as a single-chain polypeptide that is proteolytically processed to yield the two chains of mature GPIIb present on the cell surface. Analysis of the amino acid sequence surrounding the proposed light-heavy chain junction of GPIIb suggests a second potential site following a pair of basic residues 12-15 residues upstream from the reported amino terminus of the light chain. We have utilized anti-peptide antibodies to examine the possibility of alternative cleavage at these two potential sites. Peptide V43 precedes the dibasic sequence and is known to reside in the heavy chain. Peptide V41 contains the sequence between the two potential sites. In immunoblots, anti-V43 reacted only with the heavy chain while anti-V41 reacted only with the light chain. Immunoprecipitation of surface-labeled platelets indicated 97% of the GPIIb light chain contains the V41 sequence while approximately 3% of GPIIb molecules lack the V41 sequence on both the light and heavy chains. These data indicate that GPIIb is primarily cleaved 12-15 amino acids upstream from the reported amino terminus of the light chain while in a minor proportion of GPIIb molecules cleavage occurs at both sites.
Insights
Platelet glycoprotein IIb undergoes primary cleavage near its light chain terminus. A minor fraction exhibits alternative cleavage, indicating dual processing sites for this crucial platelet receptor component.
Area of Science:
- Biochemistry
- Molecular Biology
- Cell Biology
Background:
- Platelet membrane glycoprotein (GP) IIb-IIIa is a key receptor for adhesive proteins like fibrinogen.
- GPIIb is synthesized as a single chain and processed into heavy and light chains on the cell surface.
Purpose of the Study:
- To investigate alternative cleavage sites in the processing of platelet GPIIb.
- To characterize the processing heterogeneity of GPIIb using anti-peptide antibodies.
Main Methods:
- Utilized anti-peptide antibodies (anti-V43 and anti-V41) to probe GPIIb processing.
- Performed immunoblots and immunoprecipitation of surface-labeled platelets.
Main Results:
- Anti-V43 reacted with the heavy chain; anti-V41 reacted with the light chain.
- 97% of GPIIb light chains contained the V41 sequence, indicating primary cleavage site.
- Approximately 3% of GPIIb molecules showed alternative cleavage at both potential sites.
Conclusions:
- GPIIb is predominantly cleaved 12-15 amino acids upstream of the reported light chain terminus.
- A minor population of GPIIb molecules undergoes cleavage at an alternative site, suggesting dual processing pathways.