Fig. 6: The structure of Bcs3 in complex with DP2.

a, Surface representation of a selected dimer of Bcs3-DP2, showing two DP2 molecules depicted as cyan surfaces and each located in a concave grove of an SH3b domain. DP2 faces the common main groove of the Bcs3 dimer with its nonreducing end, at which group 2 capsule polymers are elongated³. b, The reducing end of DP2 is facing away from the main grove of the concave face. It is tempting to speculate that this arrangement allows the polymer to exit the basket-like compartment during elongation. c, View similar to b but the front protomer is depicted as ribbons. d, Corresponding Coulombic surface charge distribution. e, Detail of the SH3b domain in complex with DP2 showing the two β-sheets and the loops forming the polymer-binding groove. The first β-sheet is composed of β27, β28 and β32; the second β-sheet comprises β29, β30 and β31 (beta sheets numbered in Supplementary Data 1). The DP2 molecule accommodates into a groove defined by β30 and β31, the loops connecting β27-β28 (loop 1, the so-called RT loop in SH3 domains²⁸; residues 1,056–1,079, tan), β29-β30 (loop 2; residues 1,091–1,095, dark green) and β31-β32 (loop 3; residues 1,107–1,111, red), as well as the loop connecting SH3b to the third α-helical bundle (HB-3) of CriT (loop 4; residues 1,031–1,038, orange). Residues implicated in the interactions with DP2 are depicted. f,g, Surface representation of a selected SH3b domain showing the overall fist-like shape comprising a central groove decorated by positive and neutral patches that hold the polymer in position. h, The superposition of all four DP2 polymer fragments belonging to all four SH3b domains shows a common overall orientation and binding mode of the ligand.