Selective iodination and polypeptide composition of pinocytic vesicles

Area of Science:

• Cell Biology
• Biochemistry
• Immunology

Background:

• Pinocytic vesicles (PVs) are crucial for cellular uptake.
• Understanding the molecular composition of PV membranes is essential for cell biology research.
• Existing methods for labeling cellular components lack specificity for internal vesicles.

Purpose of the Study:

• To develop a specific method for radioiodinating the luminal surface of pinocytic vesicles (PVs).
• To investigate the polypeptide composition of PV membranes.
• To compare the molecular makeup of PV membranes with the plasma membrane (PM).

Main Methods:

• Simultaneous endocytosis of lactoperoxidase (LPO) and glucose oxidase (GO) into macrophage cell line J774.
• Specific radioiodination of interiorized PVs using 125I and glucose at 4°C after a 37°C incubation.
• Electron microscopy (EM) cytochemistry and autoradiography for localization.
• Quantitative immunoprecipitation and SDS-PAGE for polypeptide analysis.

Main Results:

• LPO and GO were interiorized via fluid-phase pinocytosis, localized to PVs, and did not bind to the plasma membrane.
• A 5-minute incubation at 37°C followed by iodination at 4°C selectively labeled PVs.
• Radioiodination incorporated iodide into PV membrane and content polypeptides.
• PV membrane proteins showed striking similarity to PM proteins via SDS-PAGE and immunoprecipitation.
• Key membrane antigens, including Fc receptor and H-2Dd, were similarly iodinated in both PV and PM.

Conclusions:

• The developed method allows for specific radioiodination of pinocytic vesicles.
• PV membrane is derived from a representative sample of plasma membrane polypeptides.
• This technique provides insights into the biogenesis and composition of intracellular vesicles.