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Microtubule-associated protein-dependent binding of phagosomes to microtubules
A Blocker1, F F Severin, A Habermann
1Cell Biology Programme, European Molecular Biology Laboratory, Meyerhofstrasse 1, 69117 Heidelberg, Germany.
Abstract:
In macrophages, phagosome movement is microtubule-dependent. Microtubules are a prerequisite for phagosome maturation because they facilitate interactions between phagosomes and organelles of the endocytic pathway. We have established an in vitro assay that measures the binding of purified phagosomes to microtubules. This binding depends on the presence of membrane proteins, most likely integral to the surface of phagosomes, and on macrophage cytosol. The cytosolic binding factor can interact with microtubules prior to the addition of phagosomes to the assay, suggesting that it is a microtubule-associated protein (MAP). Consistent with this, depletion of MAPs from the cytosol by microtubule affinity removes all binding activity. Microtubule motor proteins show no binding activity, whereas a crude MAP preparation is sufficient to support binding and to restore full binding activity to MAP-depleted cytosol. We show that the activating MAP factor is a heat-sensitive protein(s) that migrates at around 150 kDa by gel filtration.
Insights
This study identifies a heat-sensitive microtubule-associated protein (MAP) crucial for phagosome binding to microtubules in macrophages. This MAP is essential for phagosome maturation and endocytic pathway interactions.
Area of Science:
- Cell Biology
- Immunology
- Cytoskeletal Dynamics
Background:
- Phagosome movement in macrophages relies on microtubules, which are vital for phagosome maturation and interactions within the endocytic pathway.
- Understanding the molecular mechanisms governing phagosome-microtubule interactions is key to elucidating phagosome maturation processes.
Purpose of the Study:
- To establish an in vitro assay for measuring phagosome-microtubule binding.
- To identify the molecular factors responsible for mediating this binding interaction.
Main Methods:
- Development of an in vitro assay to quantify purified phagosome binding to microtubules.
- Fractionation of macrophage cytosol and testing for microtubule-binding activity.
- Depletion of microtubule-associated proteins (MAPs) from cytosol using microtubule affinity.
- Analysis of protein size using gel filtration.
Main Results:
- Phagosome-microtubule binding requires both phagosome membrane proteins and macrophage cytosol.
- A cytosolic factor, identified as a microtubule-associated protein (MAP), mediates this binding.
- Depletion of MAPs abolished binding activity, while MAP-rich preparations restored it.
- The key MAP factor is heat-sensitive and approximately 150 kDa.
Conclusions:
- Macrophage phagosome-microtubule binding is mediated by specific MAPs, not motor proteins.
- This MAP-dependent interaction is critical for facilitating phagosome maturation and endocytic pathway organelle interactions.
- The identified MAP represents a novel target for understanding phagosome trafficking and function.