Other Identified PcG Proteins and Partners

The other characterized members still remain to be assigned to any biochemical complex, or to a specific function in the framework of the known complexes. The Pleiohomeotic protein (PHO) and, more recently, the PHO-like product (PHOL) are the only members of the PcG shown to bind to DNA at a specific DNA binding motif (Brown et al. 1998,2003; Fritsch et al. 1999). Consistent with the high degree of conservation in the Zinc finger domain responsible for DNA binding between PHO and PHOL, it was shown that PHOL binds to the same DNA sequences as PHO (Brown et al. 2003), suggesting a redundant function for these two factors.

In the early embryo, at a stage where PcG repression still does not occur, the PC, PH, ESC, E(Z), PHO, Rpd3 and the GAGA Factor (GAF, a member of the trxG) proteins interact (Poux et al. 2001b). This work provides evidence for an early physical interaction between members of the two known PcG complexes and members not yet assigned to any of them and it suggests that contacts between these components may be important for establishment of PcG-medi-ated silencing. Moreover, the PHO protein was shown to be capable of direct interaction with the PC protein (Mohd-Sarip et al. 2002), providing a link between a sequence-specific PcG member and PRC1. Although YY1, the human PHO homologue was shown to interact with EED, a human ESC homologue (Satijn et al. 2001), no such interaction was found in Drosophila, thus leaving open the question whether PHO may link both complexes via a joint recruitment. The function of some other proteins that were originally assigned to the PcG is even less understood. For instance, the two proteins E(PC) and ASX (Table 1) might be classified in the PcG genetically, and on the basis of their partial co-localization with PC in polytene chromosome staining experiments (Sinclair et al. 1998a,b). However, ASX seems to have a dual function, as some mutations in the Asx gene also enhance trxG phenotypes, and since this protein has tissue-specific functions via specific cofactors (Dietrich et al. 2001). Moreover, both ASX and E(PC) proteins interfere with the phenomenon of Position Effect Variegation, a gene silencing phenomenon that depends on heterochromatin components and does not involve the other members of the PcG (Sinclair et al. 1998b). Thus, the molecular dissection of the function of these two factors, and perhaps other proteins associated with the PcG, may uncover intriguing links between PcG members and other cellular functions.

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