Fig. 1: Structural organization and phase separation of Bik1.

A One possible structure of the Bik1 dimer as predicted by AlphaFold (for alternative predicted structures, see Supplementary Fig. 1A) with color-coded domains. The three predicted coiled-coil segments A, B, and C of the coiled-coil domain, which are separated by linkers are indicated. B Charge distribution on Bik1’s surface. Positively charged patches are colored in blue, negatively charged patches are in red, and neutral patches are in light gray. C Bik1 truncation mutants used in this study with their respective designations and residue boundaries. CG, CAP-Gly domain; L1 and L2, linker regions 1 and 2; FL, full-length Bik1; ΔN, Bik1 variant lacking the N-terminal CAP-Gly domain and the linker L1; ZnF, zinc finger; QQFF, peptide Gln-Gln-Phe-Phe; ΔNC, Bik1 variant lacking both the N- and C-terminal regions and corresponding to Bik1’s coiled-coil domain. D Phase diagram of N-terminally His-tagged Bik1 FL in 20 mM Tris-HCl, pH 7.4, supplemented with 1 mM DTT and 10% glycerol. E Confocal fluorescence microscopy images of micrometer-sized droplets observed for 40 µM Bik1 (mixture of 90% Bik1 FL and 10% mNG-Bik1 FL; both proteins are N-terminally His-tagged) in a buffer consisting of 20 mM Tris-HCl, pH 7.4, supplemented with 250 mM NaCl and 2% glycerol. View from the top (left) and a mid-plane slice (right). F The average intensity (N = 233) of mNG throughout mNG-Bik1 FL droplets is shown in (E). Error bars represent standard errors. G Possible inter-molecular interactions between Bik1 dimers mediated by the protein’s N-terminal CAP-Gly domain and C-terminal EEY/F-like motif. Unstructured regions of the protein are shown as blue solid lines. CAP-Gly, coiled coil, and zinc-finger domains are depicted in the same way as in (C). Source data of D and F are provided in the Source Data file.