Ci protein, short for Cubitus interruptus, is a zinc finger containing transcription factor involved in the Hedgehog signaling pathway. In the absence of a signal to the Hedgehog signaling pathway, the Ci protein is cleaved and destroyed in proteasomes. It isn't, however, completely destroyed; part of the protein survives and acts as a repressor in the nucleus, keeping genes responsive to the Hedgehog signal silent.
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Degradation of Ci
The degradation of Ci protein depends on a large multiprotein complex, which contains a serine/threonine kinase of unknown function, an anchoring protein that binds to microtubules (to keep the Ci protein out of the nucleus) and an adaptor protein. When the Hedgehog signaling pathway is turned on, the Ci proteolysis is suppressed and the unprocessed CI protein enters the nucleus, where it activates the transcription of its target genes. Ci undergoes complete or partial degradation in the cells, the detailed molecular mechanism is poorly understood. It has been reported that an AAA ATPase Ter94 complex and K11/K48 ubiquitin chains are involved in the selection of Ci degradation.