Fig. 2: The crystal structure HaPE560.

a Cartoon diagram showing the crystal structure of the open-braced quaternary form HaPE560 (α subunits purple, β subunits gray). b Comparison of the open-braced HaPE560 α subunit structure with that of the open form HaPE555. The open-braced α subunit is shown in purple cartoon while the open form α subunit is in green. The L1 loop can be seen in the center of the panel. c. Inset comparing the L1 protrusion (marked) and the path of the HaPE555 (green) as viewed by the eye marker in b. The chromophores of both proteins are shown in gold in b and c, with the chromophore of HaPE560 shifted away from the binding pocket. d The interaction between the L1 loop and the neighboring α chromophore. The seven residue L1 loop interacts with the α19 chromophore of the opposite α subunit in two key places: Asp53, which ligates the nitrogen of pyrrole ring A and Gly51, where the propionate from pyrrole ring B forms a hydrogen bond to the backbone nitrogen, stabilizing the β hairpin structure. e The interaction between the L1 loop and the α19 chromophore from the opposite α subunit leads to: a rotation of pyrrole ring A relative to ring B, when compared to open form structures; and an anchoring of the propionate side chain of pyrrole ring B, decreasing flexibility (and thus excitation decay routes). Right panel shows the complete α19 chromophore in the same orientation as d, while the left panel highlights the changes in chromophore geometry. f An alignment of open-braced form α subunit transcriptome sequences from: Ha – H. andersenii; Hr – H. rufescens; and Ht – H. tepida. Color codes are as per Fig. 1a.