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Figure 7
Details of the different NC interfaces. (a) SEPT9GC–GDP (PDB entry 5cyo) has a canonical open interface in which the PB2 region (red) does not interact with the polyacidic region (yellow) of the neighbouring subunit. The typical salt bridges involving α6 and the loop following α2 (including Arg124 from PB2) are observed (Valadares et al., 2017BB62). α2′ and α6′ refer to equivalent helices from the other subunit. (b) The closed interface of SEPT3α0G–GDP (PDB entry 4z54) shows how the PB2 region now interacts with the polyacidic region. In this structure several salt bridges between the two regions are present, but only that involving Arg162 (the homologue of Arg124 in SEPT9) is represented explicitly. There is significant rearrangement of the remaining salt bridges as a result of interface closure. (c) The closed interface of SEPT9GC–GTPyS (PDB entry 5cyp) also shows the close proximity of PB2 and the polyacidic region as a result of interface closure. In this case the subunits are slight closer together than shown in (b) and the poorer resolution prohibits a complete description of the interactions involved. (d) The shifted NC interface as observed in SEPT12G–GDP (PDB entry 6mqk). Owing to the shift of one subunit with respect to the other in a direction roughly parallel to helix α6, the interface no longer has twofold symmetry and the interactions observed on the upper side of the interface (involving PB2 and the polyacidic region) are missing on the lower side.

IUCrJ
Volume 7| Part 3| May 2020| Pages 462-479
ISSN: 2052-2525
https://doi.org/10.1107/S2052252520002973