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The mechanochemistry of endocytosis.

Liu J, Sun Y, Drubin DG, Oster GF - PLoS Biol. (2009)

Bottom Line: Although individual molecular players have been studied intensively, how they all fit into a coherent picture of endocytosis remains unclear.The central idea is that membrane curvature is coupled to the accompanying biochemical reactions.Calculated phase diagrams reproduce endocytic mutant phenotypes observed in experiments and predict unique testable endocytic phenotypes in yeast and mammalian cells.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular and Cell Biology, University of California Berkeley, Berkeley, California, United States of America.

ABSTRACT
Endocytic vesicle formation is a complex process that couples sequential protein recruitment and lipid modifications with dramatic shape transformations of the plasma membrane. Although individual molecular players have been studied intensively, how they all fit into a coherent picture of endocytosis remains unclear. That is, how the proper temporal and spatial coordination of endocytic events is achieved and what drives vesicle scission are not known. Drawing upon detailed knowledge from experiments in yeast, we develop the first integrated mechanochemical model that quantitatively recapitulates the temporal and spatial progression of endocytic events leading to vesicle scission. The central idea is that membrane curvature is coupled to the accompanying biochemical reactions. This coupling ensures that the process is robust and culminates in an interfacial force that pinches off the vesicle. Calculated phase diagrams reproduce endocytic mutant phenotypes observed in experiments and predict unique testable endocytic phenotypes in yeast and mammalian cells. The combination of experiments and theory in this work suggest a unified mechanism for endocytic vesicle formation across eukaryotes.

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Phase diagrams for endocytic dynamics.The shaded areas represent the parameter regions for successful endocytosis; the star in each phase diagram represents the parameter set used in the fitting plot in Figure 3. (A) Strength of BDP PIP2 protection: K2 versus curvature-dependent PIP2 hydrolysis rate k2; (B) Curvature-dependent factor for phosphatase recruitment rate, α versus phosphatase recruitment rate k3; (C) Relative rate of BDP dynamics versus actin polymerization rate k7; (D) Curvature-dependent factor of BDP recruitment rate χ versus interfacial force constant λ0. Each phenotype is characterized by: (a) time-lapse plot for the coat proteins (red), actin (blue), BDP (green), phosphatase (orange), and the membrane tip position (black); (b) the time course for interfacial force development (purple); (c) the time course for membrane shape change (black). The intensity of each functional module in the phenotype plots is normalized relative to the corresponding wild-type normalized intensity shown in Figure 3, thus representing the relative abundance. Phenotype 1: Without PIP2 hydrolysis [k2 reduces from 20 (nm) per second to 0]. Phenotype 2: Increased protection strength of PIP2 hydrolysis at the tubule region [ increases from  to ]. Phenotype 3: Increased phosphatase recruitment rate [α increases from 100 nm to 500 nm]. Phenotype 4: BDP recruitment does not occur.
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pbio-1000204-g006: Phase diagrams for endocytic dynamics.The shaded areas represent the parameter regions for successful endocytosis; the star in each phase diagram represents the parameter set used in the fitting plot in Figure 3. (A) Strength of BDP PIP2 protection: K2 versus curvature-dependent PIP2 hydrolysis rate k2; (B) Curvature-dependent factor for phosphatase recruitment rate, α versus phosphatase recruitment rate k3; (C) Relative rate of BDP dynamics versus actin polymerization rate k7; (D) Curvature-dependent factor of BDP recruitment rate χ versus interfacial force constant λ0. Each phenotype is characterized by: (a) time-lapse plot for the coat proteins (red), actin (blue), BDP (green), phosphatase (orange), and the membrane tip position (black); (b) the time course for interfacial force development (purple); (c) the time course for membrane shape change (black). The intensity of each functional module in the phenotype plots is normalized relative to the corresponding wild-type normalized intensity shown in Figure 3, thus representing the relative abundance. Phenotype 1: Without PIP2 hydrolysis [k2 reduces from 20 (nm) per second to 0]. Phenotype 2: Increased protection strength of PIP2 hydrolysis at the tubule region [ increases from to ]. Phenotype 3: Increased phosphatase recruitment rate [α increases from 100 nm to 500 nm]. Phenotype 4: BDP recruitment does not occur.

Mentions: In this section, we will explore in detail how mechanochemical feedback ensures the precise timing and sequence of endocytic events and guarantees rapid endocytic vesicle scission. In Figure 6A–6D phase diagrams for endocytosis are computed for different pairs of model parameters. These diagrams serve several purposes. First, they show that the model is robust: it can generate successful endocytosis over a large range of the parameters. Second, equipped with these phase diagrams, we can vary the parameters to mimic the conditions of mutant experiments. Third, they constitute an independent experimental test of the model. This is because the identities of the functional modules were in part derived from mutant experiments, but we did not explicitly take into account the mutant phenotypes in the model. That is, we used the five time-lapse curves and membrane shape changes to determine the four free parameters in the model, and then used these parameter values to predict mutant phenotypes. Thus, these predictions are independent of the parameter set, and consequently the agreement between predicted and observed phenotypes constitutes cross-validation of the model. Finally, based on the calculated phase diagrams, we can predict endocytic phenotypes for mutants that have not yet been made, thus guiding further experiments.


The mechanochemistry of endocytosis.

Liu J, Sun Y, Drubin DG, Oster GF - PLoS Biol. (2009)

Phase diagrams for endocytic dynamics.The shaded areas represent the parameter regions for successful endocytosis; the star in each phase diagram represents the parameter set used in the fitting plot in Figure 3. (A) Strength of BDP PIP2 protection: K2 versus curvature-dependent PIP2 hydrolysis rate k2; (B) Curvature-dependent factor for phosphatase recruitment rate, α versus phosphatase recruitment rate k3; (C) Relative rate of BDP dynamics versus actin polymerization rate k7; (D) Curvature-dependent factor of BDP recruitment rate χ versus interfacial force constant λ0. Each phenotype is characterized by: (a) time-lapse plot for the coat proteins (red), actin (blue), BDP (green), phosphatase (orange), and the membrane tip position (black); (b) the time course for interfacial force development (purple); (c) the time course for membrane shape change (black). The intensity of each functional module in the phenotype plots is normalized relative to the corresponding wild-type normalized intensity shown in Figure 3, thus representing the relative abundance. Phenotype 1: Without PIP2 hydrolysis [k2 reduces from 20 (nm) per second to 0]. Phenotype 2: Increased protection strength of PIP2 hydrolysis at the tubule region [ increases from  to ]. Phenotype 3: Increased phosphatase recruitment rate [α increases from 100 nm to 500 nm]. Phenotype 4: BDP recruitment does not occur.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2742711&req=5

pbio-1000204-g006: Phase diagrams for endocytic dynamics.The shaded areas represent the parameter regions for successful endocytosis; the star in each phase diagram represents the parameter set used in the fitting plot in Figure 3. (A) Strength of BDP PIP2 protection: K2 versus curvature-dependent PIP2 hydrolysis rate k2; (B) Curvature-dependent factor for phosphatase recruitment rate, α versus phosphatase recruitment rate k3; (C) Relative rate of BDP dynamics versus actin polymerization rate k7; (D) Curvature-dependent factor of BDP recruitment rate χ versus interfacial force constant λ0. Each phenotype is characterized by: (a) time-lapse plot for the coat proteins (red), actin (blue), BDP (green), phosphatase (orange), and the membrane tip position (black); (b) the time course for interfacial force development (purple); (c) the time course for membrane shape change (black). The intensity of each functional module in the phenotype plots is normalized relative to the corresponding wild-type normalized intensity shown in Figure 3, thus representing the relative abundance. Phenotype 1: Without PIP2 hydrolysis [k2 reduces from 20 (nm) per second to 0]. Phenotype 2: Increased protection strength of PIP2 hydrolysis at the tubule region [ increases from to ]. Phenotype 3: Increased phosphatase recruitment rate [α increases from 100 nm to 500 nm]. Phenotype 4: BDP recruitment does not occur.
Mentions: In this section, we will explore in detail how mechanochemical feedback ensures the precise timing and sequence of endocytic events and guarantees rapid endocytic vesicle scission. In Figure 6A–6D phase diagrams for endocytosis are computed for different pairs of model parameters. These diagrams serve several purposes. First, they show that the model is robust: it can generate successful endocytosis over a large range of the parameters. Second, equipped with these phase diagrams, we can vary the parameters to mimic the conditions of mutant experiments. Third, they constitute an independent experimental test of the model. This is because the identities of the functional modules were in part derived from mutant experiments, but we did not explicitly take into account the mutant phenotypes in the model. That is, we used the five time-lapse curves and membrane shape changes to determine the four free parameters in the model, and then used these parameter values to predict mutant phenotypes. Thus, these predictions are independent of the parameter set, and consequently the agreement between predicted and observed phenotypes constitutes cross-validation of the model. Finally, based on the calculated phase diagrams, we can predict endocytic phenotypes for mutants that have not yet been made, thus guiding further experiments.

Bottom Line: Although individual molecular players have been studied intensively, how they all fit into a coherent picture of endocytosis remains unclear.The central idea is that membrane curvature is coupled to the accompanying biochemical reactions.Calculated phase diagrams reproduce endocytic mutant phenotypes observed in experiments and predict unique testable endocytic phenotypes in yeast and mammalian cells.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular and Cell Biology, University of California Berkeley, Berkeley, California, United States of America.

ABSTRACT
Endocytic vesicle formation is a complex process that couples sequential protein recruitment and lipid modifications with dramatic shape transformations of the plasma membrane. Although individual molecular players have been studied intensively, how they all fit into a coherent picture of endocytosis remains unclear. That is, how the proper temporal and spatial coordination of endocytic events is achieved and what drives vesicle scission are not known. Drawing upon detailed knowledge from experiments in yeast, we develop the first integrated mechanochemical model that quantitatively recapitulates the temporal and spatial progression of endocytic events leading to vesicle scission. The central idea is that membrane curvature is coupled to the accompanying biochemical reactions. This coupling ensures that the process is robust and culminates in an interfacial force that pinches off the vesicle. Calculated phase diagrams reproduce endocytic mutant phenotypes observed in experiments and predict unique testable endocytic phenotypes in yeast and mammalian cells. The combination of experiments and theory in this work suggest a unified mechanism for endocytic vesicle formation across eukaryotes.

Show MeSH
Related in: MedlinePlus