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Fig. 1 bOpto-BMP and -Nodal signaling activation strategy. (A) The endogenous BMP signaling pathway is activated by BMP ligand binding, leading to formation of a type I/II receptor complex, phosphorylation of Smad1/5/9, and expression of BMP target genes. The type I receptors BMPR1aa and Acvr1l are also known as Alk3 and Alk8, respectively. BMPR2a is a type II receptor. (B) bOpto-BMP constructs38. Putative kinase domains from BMPR1aa and Acvr1l are fused to LOV; the BMPR2a-LOV fusion contains the putative kinase domain and receptor C-terminal domain (CTD). All fusions are membrane-targeted with a myristoylation motif (Myr). Domains are separated by glycine-serine (GS) linkers. Constructs are tagged at the CTD with an HA epitope tag. This combination of three constructs was found to optimally activate BMP signaling. (C) bOpto-BMP-mediated BMP signaling activation. When exposed to blue light, LOV domains dimerize, which is thought to trigger complex formation and signaling activation. (D) The endogenous Nodal signaling pathway is activated by Nodal ligand binding, leading to formation of a type I/II receptor complex, phosphorylation of Smad2/3, and expression of Nodal target genes. The type I receptor Acvr1ba and the type II receptor Acvr2ba are also known as Acvr1b and Acvr2b, respectively. (E) bOpto-Nodal constructs39. Putative kinase domains from Acvr1ba and Acvr2ba are fused to LOV. All fusions are membrane-targeted with a myristoylation motif (Myr). Domains are separated by GS linkers. Constructs are tagged at the CTD with an HA epitope tag. (F) bOpto-Nodal-mediated Nodal signaling activation. When exposed to blue light, LOV domains dimerize, which is thought to trigger complex formation and signaling activation.