Fig. 5

Structural changes involved in InvG gate opening. a Comparison of the InvG secretin gate open state (colored as per Fig. 1) and the closed state (gray transparent). The major structural changes are the ordering of the N3 domain variable loop (residues 217–226 and 252–265, disordered in other secretin structures) and accompanying change in N3 domain position, the opening of the periplasmic gate involving the repositioning of GATE1 and GATE2, and the more elevated lip region caused by interactions with GATE1 and the insertion of the assembled needle filament. b and c–side view–and d and e inside view–compare the interface between outer and inner β-barrels in the open and closed states, respectively with accompanying interface areas of 2111 Ų and 1153 Ų, residues forming the open and closed interfaces shown as sticks in b and c. The core interface at the base of the β-sandwich formed by the outer and inner β-barrels is predominantly hydrophobic while the region formed by the extension of the GATE1 and GATE2 hairpins is mostly polar in nature. Key interactions defining the open and closed conformations between the N3 domain β-INSERTION, the inner β-barrel GATE1 and GATE2 motifs, and the upper outer β-barrel and lip are labeled and shown as ball and stick. The middle subunit is colored gray in d and e to define the outer and inner β-barrels. The closed gate conformation is supported by interactions of the N3 β-INSERTION, specifically Arg₁₉₈, with the kinked regions of the GATE1 (Asp₃₈₄) and GATE2 (Glu₄₂₉) hairpins and further supported by the surrounding network of interactions. In the open gate conformation, the N3 β-INSERTION interface is disrupted and the gate forming GATE1 hairpin is now extended toward the lip and packed against the outer β-barrel with Ile₃₉₄ at the tip packing in a hydrophobic notch formed by the side chains of Arg₃₂₀, Asn₃₄₀, Asn₃₅₇, and Leu₃₅₉. The GATE2 hairpin undergoes a significant rotation with residues stabilizing the closed gate—Leu₄₄₇, Pro₄₄₈, Glu₄₄₉, and Val₄₅₀—packing against the outer β-barrel wall. A salt bridge between GATE2 Glu₄₄₉ and GATE1 Arg₃₈₇ is maintained between closed and open conformations