The kinesin-related protein Costal2 associates with membranes in a Hedgehog-sensitive, smoothened-independent manner

In Drosophila, Hedgehog (Hh) signal transduction has been shown to require a multiprotein complex (Hedgehog signaling complex (HSC)), which includes the Kinesin-related protein Costal2 (Cos2), the serine/threonine protein kinase Fused (Fu), and the transcription factor Cubitus interruptus (Ci). We present evidence that a biologically relevant fraction of the HSC is found in association with cellular membranes. We demonstrate that Cos2 is capable of tethering an exogenous protein to vesicular membranes and that Cos2 association with membranes is Hh-sensitive. In addition, we demonstrate that Cos2 associates with membranes in cells that lack the transmembrane protein Smoothened (Smo) through a domain of Cos2 distinct from its recently characterized Smo binding domain. We suggest that an Hh-regulated membrane binding activity of Cos2 is part of the mechanism by which Cos2 contributes to Hh signaling. We propose a model in which there are two distinct HSCs with discrete subcellular localizations and activities: one is endosome-associated and facilitates production of a repressor form of Ci (HSC-R), and one is Smo-associated and promotes Ci activation (HSC-A). In response to Hh and through interaction with Cos2, Smo mediates both inhibition of the endosome-associated HSC-R and activation of HSC-A at the plasma membrane.