Sperm Chromatin Decondensation by Template Activating Factor I through Direct Interaction with Basic Proteins

Abstract

Template activating factor I (TAF-I) was originally identified as a host factor required for DNA replication and transcription of adenovirus genome complexed with viral basic proteins. Purified TAF-I was shown to bind to core histones and stimulate transcription from nucleosomal templates. Human TAF-I consists of two acidic proteins, TAF-Iα and TAF-Iβ, which differ from each other only in their amino-terminal regions. Here, we report that TAF-I decondenses demembraned Xenopus sperm chromatin. Human TAF-Iβ has a chromatin decondensation activity comparable to that of NAP-I, another histone binding protein, whereas TAF-Iα has only a weak activity. Analysis of molecular mechanisms underlying the chromatin decondensation by TAF-I revealed that TAF-I interacts directly with sperm basic proteins. Deletion of the TAF-I carboxyl-terminal acidic region abolishes the decondensation activity. Interestingly, the acidic region itself is not sufficient for decondensation, since an amino acid substitution mutant in the dimerization domain of TAF-I which has the intact acidic region does not support chromatin decondensation. We detected the β form of TAF-I in Xenopus oocytes and eggs by immunoblotting, and the cloning of its cDNA led us to conclude that Xenopus TAF-Iβ also decondenses sperm chromatin. These results suggest that TAF-I plays a role in remodeling higher-order chromatin structure as well as nucleosomal structure through direct interaction with chromatin basic proteins.

ACKNOWLEDGMENTS

We thank Yukio Ishimi for providing anti-NAP-I antibody and Yoshihiro Yoneda and Naoko Imamoto for anti-nucleoplasmin antibody. We are also grateful to Fumio Hanaoka for support at the initial stage of this work, Takeshi Mizuno for help with the fluorescence microscopy, and Keita Ohsumi for useful discussions.

This work was supported in part by grants-in-aid from the Ministry of Education, Science, Sports and Culture of Japan, by a grant for a Biodesign Research Program from RIKEN, and by a grant from the Naito Foundation.