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
proteasome
Proteasomes are protein complexes which degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks peptide bonds. Enzymes that help such reactions are called proteases.
Proteasomes are part of a major mechanism by w ...
s. It isn't, however, completely destroyed; part of the protein survives and acts as a
repressor
In molecular genetics, a repressor is a DNA- or RNA-binding protein that inhibits the expression of one or more genes by binding to the operator or associated silencers. A DNA-binding repressor blocks the attachment of RNA polymerase to the ...
in the
nucleus, keeping genes responsive to the Hedgehog signal silent.
Degradation of Ci
The degradation of Ci protein depends on a large multiprotein complex, which contains a serine/threonine
kinase
In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...
of unknown function, an
anchoring protein that binds to
microtubule
Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 an ...
s (to keep the Ci protein out of the nucleus) and an adaptor protein.
When the Hedgehog signaling pathway is turned on, the Ci
proteolysis
Proteolysis is the breakdown of proteins into smaller polypeptides or amino acids. Uncatalysed, the hydrolysis of peptide bonds is extremely slow, taking hundreds of years. Proteolysis is typically catalysed by cellular enzymes called protease ...
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.
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
Negative feedback (or balancing feedback) occurs when some function (Mathematics), function of the output of a system, process, or mechanism is feedback, fed back in a manner that tends to reduce the fluctuations in the output, whether caused by ...
, since the resulting increase in Patched protein on the cell surface inhibits the Hedgehog pathway.
References
External links
*
''Drosophila'' ''cubitus interruptus'' - The Interactive Fly
{{Transcription factors
Transcription factors
Hedgehog signaling pathway