Fig. 2: BRR2 binding by FBP21 and C9ORF78 is mutually exclusive.

a Cartoon model of the BRR2^HR-PRPF8^Jab1-C9ORF78 complex (top) and close-up of the C9ORF78 binding site at the C-terminal Sec63 module of BRR2^HR (bottom; boxed in the top image). Selected interacting residues are shown as sticks and labeled. In this and the following structure images: dashed lines, hydrogen bonds or salt bridges. b Multiple sequence alignment of C9ORF78 residues 5-58 (BRR2^HR-binding region) from different species, as indicated on the left. Residues are colored light to dark according to increasing level of conservation (54% conservation, light yellow; 100% conservation, orange). Residue numbering according to human C9ORF78 is indicated above the sequences. Secondary structure elements observed in the BRR2^HR-PRPF8^Jab1-C9ORF78 cryoEM structure are indicated below the sequence. Residues R9 (reduced BRR2 binding upon alanine exchange) and R41 (abrogation of BRR2 binding upon alanine exchange) are highlighted by triangles. The alignment was prepared with Homologene (NCBI) employing Clustal Omega⁷⁵ and shaded with ALSCRIPT⁷⁶. c Cartoon model of the BRR2^HR-FBP21^200-376 complex (top) and close-up of the FBP21^200-376 binding site at the C-terminal Sec63 module of BRR2^HR (bottom; boxed in the top image). Selected interacting residues are shown as sticks and labeled. d SDS-PAGE gels showing elution fractions from analytical SEC, monitoring competitive binding of FBP21^116-376 and C9ORF78 to a BRR2^HR-PRPF8^Jab1 complex. BRR2^HR, PRPF8^Jab1 and FBP21^116-376 were pre-incubated and complex formation was analyzed without (top) or with (bottom) subsequent addition of GST-C9ORF78. Independent experiments were conducted twice with similar results. Source data are provided as a Source Data file.