Coordinated Translocation of Mammalian Gli Proteins and Suppressor of Fused to the Primary Cilium

Huiqing Zeng,Jinping Jia,Aimin Liu,Rafael Linden

doi:10.1371/journal.pone.0015900

Huiqing Zeng, Jinping Jia + Show 2 more

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https://doi.org/10.1371/journal.pone.0015900

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Journal: PloS one	Publication Date: Dec 29, 2010
Citations: 122	License type: CC BY 4.0

Affiliation: Pennsylvania State University

Abstract

Intracellular transduction of Hedgehog (Hh) signals in mammals requires functional primary cilia. The Hh signaling effectors, the Gli family of transcription factors, and their negative regulator, Suppressor of Fused (Sufu), accumulate at the tips of cilia; however, the molecular mechanism regulating this localization remains elusive. In the current study, we show that the ciliary localization of mammalian Gli proteins depends on both their N-terminal domains and a central region lying C-terminal to the zinc-finger DNA-binding domains. Invertebrate Gli homologs Ci and Tra1, when over-expressed in ciliated mouse fibroblasts, fail to localize to the cilia, suggesting the lack of a vertebrate-specific structural feature required for ciliary localization. We further show that activation of protein kinase A (PKA) efficiently inhibits ciliary localization of Gli2 and Gli3, but only moderately affects the ciliary localization of Gli1. Interestingly, variants of Gli2 mimicking the phosphorylated or non-phosphorylated states of Gli2 are both localized to the cilia, and their ciliary localizations are subjected to the inhibitory effect of PKA activation, suggesting a likely indirect mechanism underlying the roles of PKA in Gli ciliary localization. Finally, we show that ciliary localization of Sufu is dependent on ciliary-localized Gli proteins, and is inhibited by PKA activation, suggesting a coordinated mechanism for the ciliary translocation of Sufu and Gli proteins.

Highlights

Hedgehog (Hh) family of secreted proteins play pivotal roles in development, adult stem cell maintenance and cancers [1]
Further truncation of the C-terminus completely abolishes the ciliary-localization of Gli2, suggesting that this region (647–967) constitutes at least part of the domain that mediates the ciliary localization of Gli2 (Fig. 1E; Table 1)
Consistent with the evolutionary divergence in regulation of Hh signaling between vertebrates and invertebrates, we found that Drosophila and C. elegans Gli homologues are not localized to the cilia when expressed in ciliated mouse fibroblasts

Summary

Introduction

Hedgehog (Hh) family of secreted proteins play pivotal roles in development, adult stem cell maintenance and cancers [1]. In Drosophila, Hh elicits transcriptional responses in target cells through a signal transduction pathway comprising its receptor Patched (Ptc), a serpentine receptor-like protein Smoothened (Smo), and a Hh signaling complex comprising a Fused kinase (Fu), a kinesin-like Costal (Cos2) and a transcription factor Cubitus interruptus (Ci). Ci is a dual-functional transcription factor, which, in the absence of Hh, is proteolytically processed into a transcriptional repressor. In the presence of Hh, full-length Ci is converted into a transcriptional activator that mediates the transcriptional responses of Hh target cells. The primary cilium, a surface organelle that is not present in most Drosophila cells, plays an essential role in mammalian Hh signaling [3]. Detailed genetic analyses suggest that both the transcriptional activator and repressor functions of Gli proteins are compromised in mutant mouse embryos with defective cilia [4,5,6,7]. Whether cilia are essential for the activation of all three Gli proteins remains controversial because over-expression of Gli proteins, especially Gli, is able to activate a Hh-responsive reporter gene in cultured cells independent of cilia [4,8,9]

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