Cca3, the mRNA level of which transiently decreases before initiation of DNA synthesis in regenerating rat liver cells

Identification and in silico analysis of a new group of double-histone fold-containing proteins

The double-histone fold is a rare protein fold in which two consecutive regions characterized by the typical structure of histones assemble together, thus giving a histone pseudodimer. Previously, this fold was found in a few prokaryotic histones and in the regulatory region of guanine-nucleotide exchange factors of the Sos family. Standard methods of sequence comparison did not allow us to find new proteins containing a histone pseudodimer, as previously reported (Sondermann et al. 2003). However, a deeper investigation of protein sequences showed that the two histone folds included in Sos proteins share significant sequence similarity with nucleosomal histones. On the basis of this observation, we applied a specific strategy of sequence-homology search, which led to the identification of a new group of histone pseudodimers in Cca3 and proteins similar to Cca3 (Cca3S). A homology model of the histone pseudodimer included in rat Cca3 was constructed. A subsequent structure-function relationship study revealed that the histone pseudodimers included in Cca3 and Cca3S proteins, but not those present in Sos proteins, could retain the ability of mediating protein-DNA interactions, and could consequently act as DNA-binding modules.

Integration of a growth-suppressing BTB/POZ domain protein with the DP component of the E2F transcription factor

Transcription factor E2F plays an important role in orchestrating early cell cycle progression through its ability to co-ordinate and integrate the cell cycle with the transcription apparatus. Physiological E2F arises when members of two distinct families of proteins interact as E2F-DP heterodimers, in which the E2F component mediates transcriptional activation and the physical interaction with pocket proteins, such as the tumour suppressor protein pRb. In contrast, a discrete role for the DP subunit has not been defined. We report the identification and characterization of DIP, a novel mammalian protein that can interact with the DP component of E2F. DIP was found to contain a BTB/POZ domain and shows significant identity with the Drosophila melanogaster germ cell-less gene product. In mammalian cells, DIP is distributed in a speckled pattern at the nuclear envelope region, and can direct certain DP subunits and the associated heterodimeric E2F partner into a similar pattern. DIP-dependent growth arrest is modulated by the expression of DP proteins, and mutant derivatives of DIP that are compromised in cell cycle arrest exhibit reduced binding to the DP subunit. Our study defines a new pathway of growth control that is integrated with the E2F pathway through the DP subunit of the heterodimer.