Fig. 4

The SpPBP2a minidomain is not α-helical but still interacts with SpGpsB through conserved arginines. a Arginine residues of SpPBP2a play a key role in binding to SpGpsB. Unless otherwise indicated, the fluorescence polarisation binding curves represent the interaction of TAMRA-labelled SpPBP2a_23-45 peptides with wildtype SpGpsB_1-63. The relevant dissociation constants are listed in Supplementary Table 1. b The structure of the SpGpsB_4-63:SpPBP2a_27-40 complex reveals the critical role of SpPBP2a arginines for the interaction with SpGpsB. In this cartoon, SpGpsB_4-63 is coloured cyan, and the SpPBP2a_27-40 peptide is coloured yellow (molecule 1) and green (molecule 2). The sidechains of Arg8 and Arg11 from the BsGpsB_5-64:BsPBP1_1-17 complex are shown as red sticks after a global superimposition of equivalent GpsB atoms. In molecule 1, SpPBP2a^Arg31 and SpPBP2a^Arg36 superimpose with BsGpsB_5-64^Arg8 and BsGpsB_5-64^Arg11 whereas molecule 2 accommodates SpPBP2a^Arg33 and SpPBP2a^Arg36. c, d Close-up view of the interactions of SpPBP2a from molecule 1 (c) and 2 (d) with SpGpsB_4-63. Key interfacial sidechains and backbone atoms are represented in stick format; SpGpsB_4-63 is coloured cyan and SpPBP2A_27-40 is coloured green. The van der Waals’ interactions between SpGpsB^Leu32 and SpPBP1^Arg31 (molecule 1) and SpPBP1^Arg33 (molecule 2) are in yellow. The carbonyl oxygens of SpGpsB^Ile11, SpGpsB^Phe12, SpGpsB^Glu13 and SpGpsB^Gln14 are denoted by respective red numerals