Identification of a Tetrameric Hedgehog Signaling Complex

Abstract

Hedgehog (Hh) signal transduction requires a large cytoplasmic multi-protein complex that binds microtubules in an Hh-dependent manner. Here, we show that three members of this complex, Costal2 (Cos2), Fused (Fu), and Cubitus interruptus (Ci), bind each other directly to form a trimeric complex. We demonstrate that this trimeric signaling complex exists in Drosophila lacking Suppressor of Fused (Su(fu)), an extragenic suppressor of fu, indicating that Su(fu) is not required for the formation, or apparently function, of the Hh signaling complex. However, we subsequently show that Su(fu), although not a requisite component of this complex, does form a tetrameric complex with Fu, Cos2, and Ci. This additional Su(fu)-containing Hh signaling complex does not appear to be enriched on microtubules. Additionally, we demonstrate that in response to Hh Ci accumulates in the nucleus without its various cytoplasmic binding partners, including Su(fu). We discuss a model in which Su(fu) and Cos2 each bind to Fu and Ci to exert some redundant effect on Ci such as cytoplasmic retention. This model is consistent with genetic data demonstrating that Su(fu) is not required for Hh signal transduction proper and with the elaborate genetic interactions observed among Su(fu), fu, cos2, and ci.

Hedgehog signaling promotes the degradation of tumor suppressor Sufu through the ubiquitin–proteasome pathway

“…Several key components of the Shh pathway such as Smo, Gli2 and Gli3, and Sufu have all been found to be enriched in this unique organelle, and recent data indicated that localization of Smo in primary cilia is dependent on its formation of a protein complex with barrestin and Kif3A (Kovacs et al, 2008), a kinesin-like molecule. This requirement is reminiscent of the hedgehog signaling complex of Costal-2, Fused and Ci in Drosophila (Stegman et al, 2000). As Drosophila Sufu is also a member of the hedgehog signaling complex, and there is evidence indicating that it functions to keep full-length Ci transcription activator in a stable but inactivated state, the mammalian Sufu may play an analogous role in primary cilia.…”

Regulation of Hedgehog signaling: a complex story

“…Cos2 is also involved in regulating the nuclear localization of Ci: Cos2 has been demonstrated to block Ci nuclear entry [71]. Ci that does gain access to the nucleus, does so without other members of the HSC [30], implying that Cos2, Fu and Su(fu) regulation of Ci occurs in the cytoplasm.…”

“…Thus, the direct substrates(s) of Fu have not been clearly elucidated. Fu is an integral member of both trimeric and tetrameric HSCs [29,30]. It is therefore likely that Fu targets other complex members for phosphorylation.…”

“…It is therefore likely that Fu targets other complex members for phosphorylation. The most likely substrates of Fu kinase activity are Cos2 and Su(fu), as both proteins have been found to interact directly with Fu [30,[83][84][85]. Additionally, there is some indirect evidence indicating that both Cos2 and Su(fu) are phosphorylated by Fu in response to Hh [75,79].…”

“…Drosophila with Su(fu) mutation(s) are phenotypically normal, suggesting that it is not required for functional Hh signaling [26,31,80]. However, since Su(fu) has been demonstrated to interact directly with both Fu and Ci, but not Cos2, it has been suggested that Su(fu) function may be partially redundant with that of Cos2 [30,71]. This is supported by genetic studies demonstrating that loss of cos2 suppresses fu mutations [80].…”