Functional Analysis of the SIN3-Histone Deacetylase RPD3-RbAp48-Histone H4 Connection in the Xenopus Oocyte

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Abstract
We investigated the protein associations and enzymatic requirements for the Xenopus histone deacetylase catalytic subunit RPD3 to direct transcriptional repression in Xenopus oocytes. Endogenous Xenopus RPD3 is present in nuclear and cytoplasmic pools, whereas RbAp48 and SIN3 are predominantly nuclear. We cloned Xenopus RbAp48 and SIN3 and show that expression of RPD3, but not RbAp48 or SIN3, leads to an increase in nuclear and cytoplasmic histone deacetylase activity and transcriptional repression of the TRβA promoter. This repression requires deacetylase activity and nuclear import of RPD3 mediated by a carboxy-terminal nuclear localization signal. Exogenous RPD3 is not incorporated into previously described oocyte deacetylase and ATPase complexes but cofractionates with a component of the endogenous RbAp48 in the oocyte nucleus. We show that RPD3 associates with RbAp48 through N- and C-terminal contacts and that RbAp48 also interacts with SIN3. XenopusRbAp48 selectively binds to the segment of the N-terminal tail immediately proximal to the histone fold domain of histone H4 in vivo. Exogenous RPD3 may be targeted to histones through interaction with endogenous RbAp48 to direct transcriptional repression of theXenopus TRβA promoter in the oocyte nucleus. However, the exogenous RPD3 deacetylase functions to repress transcription in the absence of a requirement for association with SIN3 or other targeted corepressors.