The PBX-Regulating Protein PREP1 is present in different PBX-complexed forms in mouse

Human PREP1, a novel homeodomain protein of the TALE super-family, forms a stable DNA-binding complex with PBX proteins in solution, a ternary complex with PBX and HOXB1 on DNA, and is able to act as a co-activator in the tra nscription of PBX-HOXB1 activated promoters (Berthelsen, J., Zappavigna, V. , Ferretti, E., Mavilio, F., Blasi, F., 1998b. The novel homeoprotein Prep1 modulates Pbx-Hox protein cooperatity. EMBO J. 17, 1434-1445; Berthelsen, J., Zappavigna, V., Mavilio, F., Blasi, F., 1998c. Prep1, a novel Functiona l partner of Pbx proteins. EMBO J. 17, 1423-1433). Here we demonstrate the presence of DNA-binding PREP I-PBX complexes also in murine cells. In vivo, PREP1 is a predominant partner of PBX proteins in various murine tissues. However, the choice of PBX family member associated with PREP1 is largely t issue-type specific. We report the cloning and expression domain of murine Prep I gene. Murine PREP I shares 100% identity with human PREP1 in the hom eodomain and 95% similarity throughout the whole protein. In the adult mous e, PREP I is expressed ubiquitously, with peaks in testis and thymus. We fu rther demonstrate the presence of murine Prep1 mRNA and protein, and of dif ferent DNA-binding PREP1-PBX complexes, in mouse embryos from at least 9.5 days p.c. Moreover, we show that PREP I is present in all embryonic tissues from at least 7.5-17.5 days p.c with a predominantly nuclear staining. PRE P1 is able to super-activate the PBX-HOXB-1 autoregulated Hoxb-1 promoter, and we show that all three proteins, PREP1, PBX and HOXB-1, are present tog ether in the mouse rhombomere 4 domain in vivo, compatible with a role of P REP1 as a regulator of PBX and HOXB-1 proteins activity during development. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.