A Unique Protection Signal in Cubitus interruptus Prevents Its Complete Proteasomal Degradation

Abstract

The limited proteolysis of Cubitus interruptus (Ci), the transcription factor for the developmentally and medically important Hedgehog (Hh) signaling pathway, triggers a critical switch between transcriptional repressor and activator forms. Ci repressor is formed when the C terminus of full-length Ci is degraded by the ubiquitin-proteasome pathway, an unusual reaction since the proteasome typically completely degrades its substrates. We show that several regions of Ci are required for generation of the repressor form: the zinc finger DNA binding domain, a single lysine residue (K750) near the degradation end point, and a 163-amino-acid region at the C terminus. Unlike other proteins that are partially degraded by the proteasome, dimerization is not a key feature of Ci processing. Using a pulse-chase assay in cultured Drosophila cells, we distinguish between regions required for initiation of degradation and those required for the protection of the Ci N terminus from degradation. We present a model whereby the zinc finger region and K750 together form a unique protection signal that prevents the complete degradation of Ci by the proteasome.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://mcb.asm.org/ .

ACKNOWLEDGMENTS

This work was supported by a Cancer Research UK studentship/project grant (C14503/A48) and a Royal Society Project grant. M.A.P. was a Wellcome Trust University Award holder when this work started.

We thank Bijal Sidapra for her contribution to Fig. 2C and Tao Zhang for help in modeling the kinetics of Ci-75 formation. We thank Alexander Soloviev, David Strutt, Martin Zeidler, Liz Smythe, Carl Smythe, and James Briscoe for useful comments on the work and the manuscript.