A unique protection signal in Cubitus interruptus prevents its complete proteasomal degradation

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.