Extended Data Fig. 9: A model of action for Bcs3.

The dimeric structural arrangement of Bcs3 allows the formation of a, a large reaction center in which the nascent chain is assembled by enzymes from both protomers - CriT/CrpP/CroT and CriT’/CrpP’/CroT’ - in a common cycle with all six enzymes working in sequential order. However, the elongated shape of the basket-like compartment, which places the CroT enzymes far away from each other, might prevent them from participating in the synthesis of a common polymeric thread. Moreover, this model would likely promote the two capsule polymers forming a coil after a few rounds of catalysis. b, Alternatively, two reaction centers are formed by different protomers - CriT/CrpP’/CroT’ and CriT’/CrpP/CroT - along the transversal longest axis across the main cleft of Bcs3. This model is supported by c, d, the fact that the active sites of the CriT/CrpP’/CroT’ and CriT’/CrpP/CroT triads are in the same plane, respectively, each separated by an angle of ca. 120°. It is worth noting that each reaction center is equidistant from the three active sites, forming a triangle (b-d, see also Extended Data Fig. 6a). The Bcs3-DP2 complex structure shows that the two chains attached to the corresponding SH3b domains each point to one of the reaction centers proposed by model 2. It is tempting to speculate that this structural arrangement of the active sites within a concave basket-like compartment promotes substrate channeling and facilitates the sequence of reactions, a characteristic of multi-enzyme complexes involved in efficient pathways.