Online supplemental material Fig. S1 shows that FH1FH2mDia2 stabilizes MTs independently of its tag. Fig. S2 shows that FH1FH2mDia2 bundles MTs in vitro. Fig. S3 shows that FH2C-mDia2 is severely deﬁcient in nucleating actin assembly in vitro. Video 1 shows time lapse of EGFP-FH1FH2mDia2 dynamics in NIH3T3 by TIRF microscopy. Video 2 shows time lapse of W853A EGFP-FH1FH2mDia2 dynamics in NIH3T3 by TIRF microscopy. Video 3 shows time lapse of I704A EGFP-FH1FH2mDia2 dynamics in NIH3T3 by TIRF microscopy. Video 4 shows time lapse of W630A EGFP-FH1FH2mDia2 dynamics in NIH3T3 by TIRF microscopy. Video 5 shows a real-time DIC microscopy video recording of axoneme-nucleated MTs undergoing dilution-induced disassembly in the presence of FH1FH2mDia2. Video 6 shows a real-time DIC microscopy video recording of axoneme-nucleated MTs undergoing dilution-induced disassembly in the presence of HKCL buffer. All movies have been compressed. High-quality uncompressed movies are available upon request. 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