2.A.6.9.2
Protein dispatched homologue 1 (Dispatched-1, DispA, DISP1) of 1521 aas and 12 TMSs.  The human ortholog (Q96F81, 1524 aas) is 83% identical.  They play roles in congenital diaphragmatic hernia (CDH) and associated pulmonary hypoplasia (PH) in humans and mice (Takahashi et al. 2018). Hedgehog (HH) signaling is essential for metazoan development. The HH ligand is secreted into the extracellular space by DISP1. Chen et al. 2020 reported the cryo-EM structure of human DISP1. It contains 12 TMSs and two extracellular domains (ECDs) like other RND homologs. Its ECDs reveal an open state, in contrast to its structural homologues PTCH1 and NPC1, whose extracellular/luminal domains adopt a closed state. The low-resolution structure of the DISP1 complex with dual lipid-modified HH ligand reveals how the ECDs of DISP1 engage with HH ligand. Several cholesterol-like molecules were found in the TMSs, implying a transport-like function of DISP1 (Chen et al. 2020). Dispatched enables tissue-patterning activity of the lipid-modified Hedgehog protein by releasing it from tightly -localized sites of embryonic expression. Wang et al. 2021 determined a cryoEM structure of the mouse DISP1, revealing three Na⁺ ions coordinated within a channel that traverses its transmembrane domain. The rate of Hedgehog export is dependent on the Na⁺ gradient across the plasma membrane. The transmembrane channel and Na⁺ binding are disrupted in DISP1-NNN, a variant with asparagine substitutions for three intramembrane aspartate residues that each coordinate and neutralize the charge of one of the three Na⁺ ions. DISP1-NNN and variants that disrupt single Na⁺ sites retain binding to, but are impaired in export of the lipid-modified Hedgehog protein to the SCUBE2 acceptor. Interaction of the amino-terminal signalling domain of the Sonic hedgehog protein (ShhN) with DISP1 occurs via an extensive buried surface area and contacts with an extended furin-cleaved DISP1 arm. Variability analysis reveals that ShhN binding is restricted to one extreme of a continuous series of DISP1 conformations. The bound and unbound DISP1 conformations display distinct Na⁺-site occupancies, which suggests a mechanism by which transmembrane Na⁺ flux may power extraction of the lipid-linked Hedgehog signal from the membrane. Na⁺-coordinating residues in DISP1 are conserved in PTCH1 and other metazoan RND family members, suggesting that Na⁺ flux powers their conformationally driven activities (Wang et al. 2021). The substrate is probably the sonic hedgehog (SHH) protein of 462 aas and one N-terminal TMS.