KAP-1 corepressor protein interacts and colocalizes with heterochromatic and euchromatic HP1 proteins: a potential role for Kruppel-associated box-zinc finger proteins in heterochromatin-mediated gene silencing

Kruppel-associated box (KRAB) domains are present in approximately one-thir d of all human zinc finger proteins (ZFPs) and are potent transcriptional r epression modules. We have previously cloned a corepressor for the KRAB dom ain, KAP-1, which is required for KRAB-mediated repression in vivo. To char acterize the repression mechanism utilized by KAP-1, we have analyzed the a bility of KAP-1 to interact with murine (M31 and M32) and human (HP1 alpha and HP1 gamma) homologues of the HPI protein family, a class of nonhistone heterochromatin-associated proteins with a well-established epigenetic gene silencing function in Drosophila, In vitro studies confirmed that KAP-1 is capable of directly interacting with M31 and hHP1 alpha, which are normall y found in centromeric heterochromatin, as well as M32 and hHP1 gamma, both of which are found in euchromatin, Mapping of the region in KAP-1 required for HPI interaction showed that amino acid substitutions which abolish HP1 binding in vitro reduce KAP-1 mediated repression in vivo. We observed col ocalization of KAP-1 with M31 and M32 in interphase nuclei, lending support to the biochemical evidence that M31 and M32 directly interact with KAP-1, The colocalization of KAP-1 with M31 is sometimes found in subnuclear terr itories of potential pericentromeric heterochromatin, whereas colocalizatio n of KAP-1 and M32 occurs in punctate euchromatic domains throughout the nu cleus. This work suggests a mechanism for the recruitment of HP1-like gene products by the KRAB-ZFP-KAP-1 complex to specific loci within the genome t hrough formation of heterochromatin-like complexes that silence gene activi ty. We speculate that gene-specific repression may be a consequence of the formation of such complexes, ultimately leading to silenced genes in newly formed heterochromatic chromosomal environments.