THE BTB POZ DOMAIN TARGETS THE LAZ3/BCL6 ONCOPROTEIN TO NUCLEAR DOTS AND MEDIATES HOMOMERISATION IN-VIVO/

The LAZ3/BCL6 gene was identified by its disruption in 3q27 translocat ions associated with diffuse large cell lymphomas, It is predicted to be a transcription factor as it contains six Kruppel-like Zinc finger motifs and a N-terminal BTB/POZ domain, a protein/protein interaction interface that is widely conserved in Metazoans, Using two antisera ra ised against non overlapping regions of the predicted ORF, we demonstr ate that the LAZ3/BCL6 protein appears as a close ca, 79 kDa doublet i n B lymphoid cell lines with either a rearranged or a non rearranged L AZ3/BCL6 locus, By immunofluorescence experiments on transiently trans fected COS-1 or NTH3T3 cells, we show that the LAZ3/BCL6 protein displ ays a punctuated nuclear localisation. This appears to rely on LAZ3/BC L6 proper folding and/or activities as it is impaired in a hormone rev ersible-fashion through fusion of LAZ3/BCL6 to the ligand-binding doma in of the oestrogen receptor, Moreover, deletion of its BTB/POZ domain leads to the disappearance of the nuclear dots although the protein r emains nuclear, In addition, by using the yeast two-hybrid system, we show that the LAZ3/BCL6 BTB/POZ domain homomerises in vivo, Thus, the LAZ3/BCL6 BTB/POZ domain has the capability to self-interact and targe t the protein to discrete nuclear substructures.