Probing intracellular motor protein activity using an inducible cargo trafficking assay

Abstract

Although purified cytoskeletal motor proteins have been studied extensively with the use of in vitro approaches, a generic approach to selectively probe actin and microtubule-based motor protein activity inside living cells is lacking. To examine specific motor activity inside living cells, we utilized the FKBP-rapalog-FRB heterodimerization system to develop an in vivo peroxisomal trafficking assay that allows inducible recruitment of exogenous and endogenous kinesin, dynein, and myosin motors to drive specific cargo transport. We demonstrate that cargo rapidly redistributes with distinct dynamics for each respective motor, and that combined (antagonistic) actions of more complex motor combinations can also be probed. Of importance, robust cargo redistribution is readily achieved by one type of motor protein and does not require the presence of opposite-polarity motors. Simultaneous live-cell imaging of microtubules and kinesin or dynein-propelled peroxisomes, combined with high-resolution particle tracking, revealed that peroxisomes frequently pause at microtubule intersections. Titration and washout experiments furthermore revealed that motor recruitment by rapalog-induced heterodimerization is dose-dependent but irreversible. Our assay directly demonstrates that robust cargo motility does not require the presence of opposite-polarity motors, and can therefore be used to characterize the motile properties of specific types of motor proteins.

Figure 1

Inducible intracellular motility assays. (A–D) Assay: a fusion construct of PEX, RFP, and FKBP targets peroxisomes. Fusions of FRB with (truncated) motor constructs (1, 2, 4, 5, 7, and 8) or adaptor protein fragments (3, 6, and 9) are recruited to FKBP and consequently the peroxisomes upon addition of rapalog. A truncated dynein fused to FKBP^∗ and FRB (5) is first dimerized by rapalog2. (E) Overview of the domain composition of the full-length versions of each motor protein or motor protein adaptor used in this study. The red line indicates the portion used to generate a truncated version fused to FRB.

Figure 2

Analyzing intracellular motility assays by radial kymography. (A and B) Peroxisome distribution before (A) and after (B) rapalog addition in the presence of KIF1A, overlaid with concentric circles (centered at the cell center). (C) Each pixel value (if above threshold) is inserted into a histogram of intensity versus distance from center (shown here for five frames, with bin size set to pixel size). (D) The histograms of all frames of the time-lapse recording can be shown as a radial kymograph that directly visualizes all changes in peroxisome distribution upon motor protein recruitment. (E) These histograms can then be used to calculate the radius required to include a given fraction of peroxisomes (i.e., 10%, 20%, …, 90%).

Figure 3

Probing specific motor protein activity using inducible intracellular motility assays. (A–F) Peroxisome distribution before (A–C) and after (D–F) rapalog addition in the presence of KIF1A (A and D), BICDN-FRB (B and E), or Myosin Vb (C and F). (G–I) Overlay of sequential binarized images from the recordings in A–F, color-coded by time as indicated in G. Blue marks the initial distribution, and red marks regions targeted upon addition of rapalog. (J–L) Time traces of the R_90% (radius of circle enclosing 90% of total cellular PEX intensity; see Fig. 2) for the indicated constructs. Rapalog is added at 0:00 min. Scale bars: 10 μm.

Figure 4

Tracing individual peroxisomes driven by specific motors. (A) Three example traces of KIF5-driven peroxisome motility. Particles often change direction by switching to a different microtubule, marked by a color change. (B) Displacement curves for the traces in A, obtained by projecting the (x,y)-coordinates onto a second- or third-order polynomial fit to the colored segments. Peroxisomes move unidirectionally over microtubules and often pause before switching to a different microtubule (marked by red arrows). (C) Histograms of instantaneous speeds for motile peroxisomes (final displacement > 1 μm) driven by KIF5 (top) or dynein (bottom). Instantaneous speeds were calculated (without sign) from the distance between consecutive (x,y) positions. The first peaks (at ∼100 nm/s, marked with black arrows) correspond to nonmotile episodes with positional fluctuations of ∼30 nm (Δt = 0.3–0.4 s). Red arrows indicate the average speed of unidirectional runs. N = 8837/5418 intervals from 533/251 trajectories for KIF5 and dynein, respectively. (D) Stills from a time-lapse recording of Kif5-driven GFP-tagged peroxisome (green) motility over microtubules (red). Yellow arrows mark three different microtubule intersections that induce pausing of the peroxisome marked with an asterisk. Motility resumes upon depolymerization of either microtubule, as evident from the white arrows that track the microtubule end. (E) Line used to generate the kymograph in panel F. (F) Kymograph showing peroxisome motility along the line shown in E. Yellow bars mark pauses induced by microtubule intersections. Unbuckling of the crossing microtubule at 9.5 s causes peroxisome deformation and apparent backward motility. (G) Kymograph of tubulin along the line shown in E reveals that the pause ending at intersections 1 and 2 coincides with depolymerization of the crossing microtubule. (H) Color merge of kymographs shown in F and G.

Figure 5

Rapalog-induced recruitment is not reversible by rapalog washout. (A) Frames from a time-lapse recording showing RFP-labeled peroxisomes in a KIF1A-FRB expressing cell. Rapalog was added at time 0:00 and washed out at time 30:00. (B) Time-coded color plot from the recording in A, in which blue and red mark distributions before (starting 15 min after Rapalog addition) and after washout, respectively. (C) Time traces of R_90% for four different cells (red). The black curve shows average ± SE; 0.1 μm rapalog was present from 0 to 30 min. (D) Recruitment of GFP-FRB (bottom) to RFP-FKBP labeled peroxisomes (top) using 1 mM rapalog present from 0 to 15 min. (E and F) Time trace of GFP intensity in regions with (black) and without (red) peroxisomes in the presence of 1 μM (E) and 10 nM (F) rapalog. Regions with peroxisomes were identified by thresholding the RFP time series. Traces were normalized by the average GFP intensity before rapalog addition in peroxisome-containing regions. Plots show average ± SE for 5 (E) and 4 (F) cells. (G) Ratio between the average GFP intensity in regions with and without peroxisomes calculated from E and F. Ratios start just above one because free cytoplasmic GFP levels are higher in the thicker perinuclear region that contains the peroxisomes. (H) Time traces of peroxisome-bound GFP intensities relative to the initial intensity of free GFP, obtained by subtracting a normalized time trace of cytoplasmic GFP intensity from the GFP intensity in peroxisome-containing regions.

Figure 6

Dose dependence of rapalog-induced motor recruitment. (A–D) Time traces of R_90% upon addition of 1 nM (A), 10 nM (B), 0.1 mM (C), and 1 mM rapalog to recruit KIF1A to peroxisomes. Red curves correspond to individual cells, black curves plot average ± SE. N = 5, 4, 10, and 15 cells, respectively. (E and F) Time traces of R_90% upon addition of 0.1 mM (E) and 1 mM (F) rapalog to cells expressing PEX-mRFP-FKBP, KIF1A-FRB, and GFP-p50. Red curves correspond to individual cells, black curves plot average ± SE. N = 4 and 13 cells, respectively. (G) Plot of all averaged traces from A–F. (H) Average time until motility onset for the curves in A–F, defined as the first time point t at which R_90%(t) exceeds R_90%__average (average R_90% before t = 0) with 10 μm. Inset shows velocities determined from the differences between the first time points at which R_90%(t) exceeds R_90%__average with 5 and 10 μm, respectively. Error bars show SE.

Figure 7

Probing the antagonistic activity of opposite polarity motor proteins using inducible intracellular motility assays. (A) Assay: a fusion construct of KIF17(1-547), GFP, and PEX26 (to allow fusion at the C-terminus of KIF17) targets peroxisomes and redistributes them throughout the cell. PEX-RFP-FKBP also targets these peroxisomes and recruits dynein through BICDN(1-594)-FRB upon addition of rapalog. (B and C) Peroxisome distribution before (B) and after (C) rapalog addition in the presence of KIF17(1-547)-GFP-PEX26 and BICDN(1-594)-FRB. (D) Overlay of sequential binarized images from the recording, color-coded by time as indicated. Blue marks the initial distribution, and red marks regions targeted upon addition of rapalog. (E) Radial kymograph for the recording in B–D, indicating the redistribution of fluorescent peroxisomes relative to the cell axis (see Fig. 2). (F) Time traces of the R_90% (see Fig. 2) versus time for three different cells. (G) Frames from a time-lapse recording showing the bidirectional motility of a peroxisome (green, visualized by GFP) upon recruitment of BICD(1-594)-FRB to peroxisomes labeled with KIF17(1-547)-GFP-PEX26 and PEX3-GFP-FKBP (acquired at 10 frames per s). (H) Two kymographs of the motility of peroxisomes extracted from recordings as in G (the upper corresponds to the example in G). (I) Kymographs showing the motility of peroxisomes extracted from recordings as in G before addition of rapalog. (J) Kymographs showing the motility of peroxisomes extracted upon recruitment of BICD(1-594)-FRB to peroxisomes labeled only with PEX3-GFP-FKBP. Scale bars: 10 μm (B) and 5 μm (G–J).