Fig. 1: The FBF-2 C-terminal tail binds to its RBD like LST-1 A and B.

Intrinsically disordered regions in a FBF-2 and b its partner protein LST-1. Schematic drawings of FBF-2 and LST-1 are shown with predicted disorder plot from IUPred2A^70,71 above and protein constructs used below. The scores in the predicted disorder plot indicate the probability of the residue being disordered. The FBF-2 CT PIM (residues 607-613) is indicated by a yellow box. The FBF-2 CT sequence is shown with its PIM highlighted yellow. FBF-2 contains a structured RBD, and LST-1 contains a predicted zinc finger (ZnF). The N-terminal region of LST-1 is sufficient for GSC self-renewal, and the C-terminal region provides feedback to limit GSC number⁷². c Crystal structure of FBF-2 RBD + CT. A ribbon diagram of the FBF-2 RBD with cylindrical helices (red) is shown in complex with cFBE RNA (tan stick model) and its CT peptide (yellow). LST-1 A (blue) and LST-1 B (green) are superimposed. d Composite 2F_o-F_c omit map contoured at 1 σ is superimposed on the crystal structure of FBF-2 RBD + CT in complex with cFBE RNA. The CT is shown as a yellow stick model. e FBF-2 RBD interacts with L610 in its CT. The FBF-2 CT peptide (yellow stick model) is shown. FBF-2 RBD residues that interact with the CT are displayed as stick models. Side chains that form a hydrophobic pocket for L610 are shown with transparent spheres. Hydrogen bond interactions between the FBF-2 RBD and backbone atoms of its CT are indicated with dashed lines. f FBF-2 interacts with core sequences in its CT, LST-1 A, and LST-1 B. Crystal structures of FBF-2 RBD + CT, FBF-2 RBD in complex with LST-1 A and LST-1 B are superimposed. The LST-1 peptides are shown as stick models (LST-1 A, blue; LST-1 B, green). FBF-2 RBD residues that interact with the peptides are shown as in e. The critical leucine residues are indicated with an asterisk.