Crossveinless d is a vitellogenin-like lipoprotein that binds BMPs and HSPGs, and is required for normal BMP signaling in the Drosophila wing

The sensitivity of the posterior crossvein in the pupal wing of Drosophila to reductions in the levels and range of BMP signaling has been used to isolate and characterize novel regulators of this pathway. We show here that crossveinless d (cv-d) mutations, which disrupt BMP signaling during the development of the posterior crossvein, mutate a lipoprotein that is similar to the vitellogenins that comprise the major constituents of yolk in animal embryos. Cv-d is made in the liver-like fat body and other tissues, and can diffuse into the pupal wing via the hemolymph. Cv-d binds to the BMPs Dpp and Gbb through its Vg domain, and to heparan sulfate proteoglycans, which are well-known for their role in BMP movement and accumulation in the wing. Cv-d acts over a long range in vivo, and does not have BMP co-receptor-like activity in vitro. We suggest that, instead, it affects the range of BMP movement in the pupal wing, probably as part of a lipid-BMP-lipoprotein complex, similar to the role proposed for the apolipophorin lipid transport proteins in Hedgehog and Wnt movement.