Supplemental Experimental Procedures Induced Expression, Imaging, and Quantification

Slide-mounted embryos were heat-shock induced by a 5 min incu-C. elegans Culture and Isolation of Mutants Wormsweremaintained, propagated, andmutagenized by standard bation at 37C, before returning to 20C. No ectopic fusions were seen in heat-shock-inducedworms or embryos lacking the hs-EFF-1methods [S1]. New ethylmethane sulfonate-induced mutations in eff-1 were isolated either in a noncomplementation screen over eff- transgene nor in hs-EFF-1-positive specimens that were not heat-shock induced. Diffusion of cell-type-specific GFP expressed from1(oj55), or in de novo screens for (1) defective larval tail morphology, (2) delayed hypodermal fusions in mid-L1 larvae, or (3) suppression the eff-1 and pal-1 promoters [S4, S5] was used to observe cell fusions in 1.5-fold (470 min postfertilization) and 100-cell (200of the lin-39 vulvaless phenotype [S2]. Mutant alleles were charac-terized by PCR sequencing from genomic DNA in whole-worm ly- min) embryos, respectively. Imaging was performed with a Zeiss LSM510 or Bio-Rad MRC600 confocal microscope with 488-nmsates. excitation. For measurements of cytoplasmic diffusion in both wild-type and EFF-1-induced embryos, fluorescence within the linearTransgenic Animals and Site-Directed Mutagenesis Plasmids constructed for this study are shown in Figure S1, and range of detection was quantified as the mean pixel intensity in an optical section through a bright cell as it fused to a dark cell. Thetheir construction is described in Plasmid Construction below. Con-structs were introduced through standard methods of microinjec- fluorescence intensities for each measured cell were then normal-ized to its prefusion and postfusion values (1 and 0, respectively),tion. pJE43 (hs-EFF-1) was coinjected with pRF4 (a dominant rol-6(su1006) marker) and either pJS191 (AJM-1::GFP) as a marker of allowing comparison of diffusion time courses among several sam-ples. Intensity measurements were made with Image