Transforming growth factor-ß-inducible early response gene 1 is a novel substrate for atypical protein kinase Cs.

The protein kinase C (PKC) family of serine/threonine kinases consists of ten different isoforms grouped into three subfamilies, denoted classical, novel and atypical PKCs (aPKCs). The aPKCs, PKCι/λ and PKCζ serve important roles during development and in processes subverted in cancer such as cell and tissue polarity, cell proliferation, differentiation and apoptosis. In an effort to identify novel interaction partners for aPKCs, we performed a yeast two-hybrid screen with the regulatory domain of PKCι/λ as bait and identified the Krüppel-like factors family protein TIEG1 as a putative interaction partner for PKCι/λ. We confirmed the interaction of both aPKCs with TIEG1 in vitro and in cells, and found that both aPKCs phosphorylate the DNA-binding domain of TIEG1 on two critical residues. Interestingly, the aPKC-mediated phosphorylation of TIEG1 affected its DNA-binding activity, subnuclear localization and transactivation potential.

Pax6 localizes to chromatin-rich territories and displays a slow nuclear mobility altered by disease mutations.

The transcription factor Pax6 is crucial for the embryogenesis of multiple organs, including the eyes, parts of the brain and the pancreas. Mutations in one allele of PAX6 lead to eye diseases including Peter's anomaly and aniridia. Here, we use fluorescence recovery after photobleaching to show that Pax6 and also other Pax family proteins display a strikingly low nuclear mobility compared to other transcriptional regulators. For Pax6, the slow mobility is largely due to the presence of two DNA-binding domains, but protein-protein interactions also contribute. Consistently, the subnuclear localization of Pax6 suggests that it interacts preferentially with chromatin-rich territories. Some aniridia-causing missense mutations in Pax6 have impaired DNA-binding affinity. Interestingly, when these mutants were analyzed by FRAP, they displayed a pronounced increased mobility compared to wild-type Pax6. Hence, our results support the conclusion that disease mutations result in proteins with impaired function because of altered DNA- and protein-interaction capabilities.

The third helix of the homeodomain of paired class homeodomain proteins acts as a recognition helix both for DNA and protein interactions.

The transcription factor Pax6 is essential for the development of the eyes and the central nervous system of vertebrates and invertebrates. Pax6 contains two DNA-binding domains; an N-terminal paired domain and a centrally located homeodomain. We have previously shown that the vertebrate paired-less isoform of Pax6 (Pax6DeltaPD), and several other homeodomain proteins, interact with the full-length isoform of Pax6 enhancing Pax6-mediated transactivation from paired domain-DNA binding sites. By mutation analyses and molecular modeling we now demonstrate that, surprisingly, the recognition helix for specific DNA binding of the homeodomains of Pax6 and Chx10 interacts with the C-terminal RED subdomain of the paired domain of Pax6. Basic residues in the recognition helix and the N-terminal arm of the homeodomain form an interaction surface that binds to an acidic patch involving residues in helices 1 and 2 of the RED subdomain. We used fluorescence resonance energy transfer assays to demonstrate such interactions between Pax6 molecules in the nuclei of living cells. Interestingly, two mutations in the homeodomain recognition helix, R57A and R58A, reduced protein-protein interactions, but not DNA binding of Pax6DeltaPD. These findings suggest a critical role for the recognition helix and N-terminal arm of the paired class homeodomain in protein-protein interactions.