The PH domain of Step is necessary but not sufficient for its membrane localization and activity.

<p>(A) Localization of GFP-Step constructs at early cellularization. N values indicate the number of embryos analyzed quantitatively. Amphiphysin (Amph) staining indicates the furrows. GFP-Step[WT] had a higher furrow membrane:cytoplasm ratio than the other constructs, as shown in the micrographs and quantifications of the ratios (means ± SD). GFP-Step[∆PH] also localized to structures that appeared to be centrosomes (yellow arrows). (B) GFP-Step[WT] over-expression produced a range of effects on furrows at early cellularization, from no effect (black) to sporadic membrane loss (red, arrows) to a general disruption of furrows including membrane loss and other disorganization (blue). Quantification of furrow defects (right) showed that the GFP-Step[∆PH] construct had a weaker effect on furrows than GFP-Step[WT] (N represents embryo numbers).