Drosophila HP1c Is Regulated by an Auto-Regulatory Feedback Loop through Its Binding Partner Woc

Jochen Abel,Axel Imhof,Elisabeth Kremmer,Ragnhild Eskeland,Grazia D Raffa,Michael J Pazin

doi:10.1371/journal.pone.0005089

Abstract

HP1 is a major component of chromatin and regulates gene expression through its binding to methylated histone H3. Most eukaryotes express at least three isoforms of HP1 with similar domain architecture. However, despite the common specificity for methylated histone H3, the three HP1 isoforms bind to different regions of the genome. Most of the studies so far focused on the HP1a isoform and its role in transcriptional regulation. As HP1a requires additional factors to bind methylated chromatin in vitro, we wondered whether another isoform might also require additional targeting factors. Indeed, we found that HP1c interacts with the DNA binding factors Woc and Row and requires Woc to become targeted to chromatin in vivo. Moreover, we show that the interaction between HP1c and Woc constitutes a transcriptional feedback loop that operates to balance the concentration of HP1c within the cell. This regulation may prevent HP1c from binding to methylated heterochromatin.

Highlights

Most eukaryotes have at least three HP1 isoforms [1], which are conserved in overall structure but bind to different regions within the genome [2]
As this modification is mostly found in transcriptionally inactive or repressed regions [8,9], HP1a is mostly considered to have a repressive function. This is further substantiated by experiments in which HP1 has artificially been targeted to an integrated promoter [10]. This view of HP1a acting merely as a repressor has been challenged by the fact that some heterochromatic genes require HP1 for active transcription [11,12]
When the hinge region is mutated, HP1 can no longer bind to chromatin in an in vitro binding assay [16]

Summary

Introduction

Most eukaryotes have at least three HP1 isoforms [1], which are conserved in overall structure but bind to different regions within the genome [2]. By performing an affinity purification using epitope-tagged HP1c we found the euchromatic isoform of HP1 in Drosophila, HP1c, binds to two Zn-finger containing DNA binding factors, Woc and Row [28], which is in marked contrast to HP1a, which does not interact with these proteins.

Results

Conclusion