The HOX homeodomain proteins block CBP histone acetyltransferase activity

Despite the identification of PBC proteins as cofactors that provide DNA af finity and binding specificity for the HOX homeodomain proteins, HOX protei ns do not demonstrate robust activity in transient-transcription assays and few authentic downstream targets have been identified for these putative t ranscription factors. During a search for additional cofactors, we establis hed that each of the 14 HOX proteins tested, from 11 separate paralog group s, binds to CBP or p300. All six isolated homeodomain fragments tested bind to CBP, suggesting that the homeodomain is a common site of interaction. S urprisingly, CBP-p300 does not form DNA binding complexes with the HOX prot eins but instead prevents their binding to DNA. The HOX proteins are not su bstrates for CBP histone acetyltransferase (HAT) but instead inhibit the ac tivity of CBP in both in vitro and in vivo systems. These mutually inhibito ry interactions are reflected by the inability of CBP to potentiate the low levels of gene activation induced by HOX proteins in a range of reporter a ssays. We propose two models for HOX protein function: (i) HOX proteins may function without CBP HAT to regulate transcription as cooperative DNA bind ing molecules with PBX, MEIS, or other cofactors, and (ii) the HOX proteins may inhibit CBP HAT activity and thus function as repressors of gene trans cription.