Molecular Properties of Zic Proteins as Transcriptional Regulators and Their Relationship to GLI Proteins

Abstract

Zic family genes encode zinc finger proteins, which play important roles in vertebrate development. The zinc finger domains are highly conserved between Zic proteins and show a notable homology to those of Gli family proteins. In this study, we investigated the functional properties of Zic proteins and their relationship to the GLI proteins. We first established an optimal binding sequence for Zic1, Zic2, and Zic3 proteins by electrophoretic mobility shift assay-based target selection and mutational analysis. The selected sequence was almost identical to the GLI binding sequence. However, the binding affinity was lower than that of GLI. Consistent results were obtained in reporter assays, in which transcriptional activation by Zic proteins was less dependent on the GLI binding sequence than GLI1. Moreover, Zic proteins activated a wide range of promoters irrespective of the presence of a GLI binding sequence. When Zic and GLI proteins were cotransfected into culturedcells,Zicproteinsenhancedorsuppressedsequencedependent, GLI-mediated transactivation depending on cell type. Taken together, these results suggest that Zic proteins may act as transcriptional coactivators and that their function may be modulated by the GLI proteins and possibly by other cell type-specific cofactors.

Emerging roles for zic genes in early development

“…The formation of Zic-Gli heterodimers is likely to preclude an interaction with DNA, as zinc fingers 3-5 in both proteins are thought to be important for DNA recognition (Pavletich and Pabo, 1993;Mizugishi et al, 2004). Accordingly, Zic proteins are reported to inhibit transcriptional activation by Gli and vice versa in Xenopus embryos and in cell culturebased reporter assays (Brewster et al, 1998;Koyabu et al, 2001;Mizugishi et al, 2001). Thus, specific interaction between Zic and Gli proteins is an attractive mechanism for regulating their respective activities in cells where they are co-expressed.…”

Adenoviral Vector-Mediated Transfer of the Indian Hedgehog Gene Modulates Lymphomyelopoiesis In Vivo

“…The lhh reporter plasmid (TK-6GBS-Luc) expressed luciferase under control of the thymidine kinase (TK) promoter and was a gift of Dr. Jun Aruga (Developmental Neurobiology Laboratory, Brain Science Institute, RIKEN, Japan) [39]. The plasmid contained six tandem copies of the GL1-target site, 5Ј-TGGGTGGTC-3Ј, inserted upstream of the TK promoter.…”

Identification of Glis1, a Novel Gli-related, Krüppel-like Zinc Finger Protein Containing Transactivation and Repressor Functions

“…ZFDs have been implicated in many functions, including DNA recognition, protein/protein interactions, transcriptional repression, and nuclear localization (52)(53)(54)(55)(56). Examination of the secondary structure of the Glis1 zinc finger domain indicated that the second half of each zinc finger consists of an ␣-helix.…”

“…The nuclear localization of Gli proteins was regulated at multiple levels that involves Sonic hedgehog signaling, nuclear import and export signals, and interactions with Fused (FU) and Suppressor of Fused (SUFUH) (60 -62). In addition, the nuclear localization of Gli proteins can be facilitated through heterodimerization with Zic proteins (56). Future studies have to ascertain the precise mechanisms that determine the nuclear transport of Glis1.…”