Ci protein

Ci protein, short for Cubitus interruptus, is a signaling protein related to 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.

Target genes

• The Wingless protein in Drosophila, which is crucial to the embryogenesis of the fruit fly, and acts through the Wnt signaling pathway.
• The Patched receptor protein of the Hedgehog signaling pathway, which production acts as a negative feedback, since the resulting increase in Patched protein on the cell surface inhibits the Hedgehog pathway.