γ-TuRCs along interphase MTs act as anticatastrophe factors. (A) Table showing the distribution of MTs in the different classes as defined in the Results and discussion section. The γ-TuRC along MTs acts as an anticatastrophe factor (53 MTs, 12 cells, and three independent experiments). (B and C) Analysis of MTs of classes I (B) and II (C). S2 cells expressing GFP–α-tubulin were imaged at the indicated times (colored dots). After cytosol extraction, cells were stained with γ-tubulin (red) and imaged again (arrowheads). The appearance of a γ-tubulin spot coincided with MT pause (class I; B), or marked a point beyond which the MT no longer depolymerized (class II; C). The graphs represent the tracking of the MT plus end extremity over time; the arrowheads indicate the position of the distal γ-tubulin spots. See Videos 4 and 5. (D) Model for the role of the γ-TuRCs bound to cytoplasmic MTs. The γ-TuRC localized along MTs induces a pause signal that could be transient, followed by MT regrowth, or long-lasting, corresponding to an arrest. Bars, 1 µm.