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Fig. 6. Arl8b stimulates lysosomal transport. (A) Still images of spinning-disc movies of lysosomes in processes of an untransfected NRK cell (con.), or of the same expressing Arl8b-GFP (Arl8b). Lysosomes were labelled by incubating cells for four hours with 1 mg/ml Alexa-Fluor-568-dextran, followed by a 20-hour chase. In both cases the cell body is at the top, and lysosomes moving toward, or away from, the cell body are indicated with open or closed triangles respectively. Bars, 5 µm. (B) Table summarizing the analysis of movies of NRK cells transfected with Arl8b-GFP, Arl8b(Q75L)-GFP or of untransfected cells. Lysosomes were labelled as in (A) and cell extensions of five cells each were imaged for three minutes at two frames per second. Individual lysosomes were tracked relative to an axis running from the cell body along the extension, and various parameters of the movement are shown. In each case a mean value is given, along with the standard error of mean. A `displacement' is defined as the movement between successive frames. A `transport event' is defined as a displacement of 0.15 µm or more per second, because this is typically the minimum speed of microtubule-dependent transport in vivo (Gross et al., 2000). A `static episode' is a period where the displacement between each successive frame does not exceed 0.15 µm/second. A `long episode of continuous fast motion' is when the net direct displacement over eight successive frames exceeds 1 µm. The length of an `episode of continuous motion' is defined for objects that have made a transport event, and is the net direct displacement before the object reverses direction relative to the axis, or does not make a transport event (i.e. moves less than 0.15 µm/second between frames). Perinuclear clusters of lysosomes were not included in the analysis as the software could not recognise every lysosome within these clusters.
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