Bruno L et al. (2011), Mechanical properties of organelles driven by m...

XB-ART-43556

PLoS One 2011 Apr 01;64:e18332. doi: 10.1371/journal.pone.0018332.

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Mechanical properties of organelles driven by microtubule-dependent molecular motors in living cells.

Bruno L , Salierno M , Wetzler DE , Despósito MA , Levi V .

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The organization of the cytoplasm is regulated by molecular motors which transport organelles and other cargoes along cytoskeleton tracks. Melanophores have pigment organelles or melanosomes that move along microtubules toward their minus and plus end by the action of cytoplasmic dynein and kinesin-2, respectively. In this work, we used single particle tracking to characterize the mechanical properties of motor-driven organelles during transport along microtubules. We tracked organelles with high temporal and spatial resolutions and characterized their dynamics perpendicular to the cytoskeleton track. The quantitative analysis of these data showed that the dynamics is due to a spring-like interaction between melanosomes and microtubules in a viscoelastic microenvironment. A model based on a generalized Langevin equation explained these observations and predicted that the stiffness measured for the motor complex acting as a linker between organelles and microtubules is ∼ one order smaller than that determined for motor proteins in vitro. This result suggests that other biomolecules involved in the interaction between motors and organelles contribute to the mechanical properties of the motor complex. We hypothesise that the high flexibility observed for the motor linker may be required to improve the efficiency of the transport driven by multiple copies of motor molecules.

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Species referenced: Xenopus laevis
Genes referenced: arhgef7 dnai1 psd trim9

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	Figure 1. Analysis of melanosomes motion perpendicular to the transport axis.(A) Representative trajectory of a melanosome moving along a microtubule. The continuous line shows the average position calculated for the microtubule. Scale bar, 200 nm (B) Motion on the perpendicular direction obtained after analyzing the trajectory represented before (C) Mean square displacement obtained from characteristic trajectory segments for the motion perpendicular to the transport direction (gray lines). The black lines show the average and standard error calculated for each data point.
	Figure 2. Analysis of melanosome position density distribution function.80 segments of trajectories corresponding to kinesin-driven melanosomes in MSH-stimulated cells were analyzed as described previously to obtain . These data were used to calculate the normalized particle density distribution function PDDF()/PDDF(0) using Δ = 1 nm. The continuous black line shows the fitting of equation (3); dotted and dashed lines represent the fitting of models corresponding to cone and r4 potentials, respectively.
	Figure 3. Power spectrum distribution of . 260 trajectory segments obtained for dynein-driven organelles during dispersion were analyzed as described to obtain the PSD. The continuous line corresponds to the fitting of PSD = af−β with β = 1.41±0.02. Dotted gray line represents the behavior expected for a pure viscous microenvironment.
	Figure 4. Scheme of the mechanical interaction between organelles and microtubules.To simplify the scheme, a single copy of the motor linker complex is represented.
	Figure 5. Distributions of κML for motor-attached melanosomes.The values for the stiffness of kinesin (squares) and dynein (circles) driven melanosomes were obtained as described in the text during dispersion (gray symbols) and aggregation (black symbols). Each histogram includes data from 150 to 200 trajectory segments, depending on the condition.
	Figure 6. Effects of p50 overexpression on melanosome dynamics.(A) Distribution of MSD1D(∞) in p50 overexpressing cells (circles). The histogram was constructed with data from 304 trajectory segments as described in the text. The distribution of MSD⊥(∞) for melanosomes driven by dynein during aggregation (squares) is also represented to make the comparison of these data easier. In both cases, the term corresponding to the error on the melanosome position determination was subtracted. Inset: representative trajectory of a melanosome in a p50 overexpressing cell. Scale bar, 100 nm. (B) Power spectrum distribution. 260 trajectory segments were analyzed as described before to obtain the PSD of either the x or y coordinate of the melanosomes. The continuous line corresponds to the fitting of a power-law behavior with β = 1.38±0.02.

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Albet-Torres, Mode of heavy meromyosin adsorption and motor function correlated with surface hydrophobicity and charge. 2007, Pubmed