369 (aa)

367 (aa); 44 (kD)

A partial peptide which results from alternative splicing and gives rise to a homeodomain that is two amino acids longer than its alternative Ipou protein.

A splice variant of the acj6 gene (previously called Twin of I-POU), has 2 additional basic residues (RK) with respect to other isoforms which allows it to specifically bind DNA. This form of the acj6 protein does not interact with vvl protein but can act as a positive transcription factor on targets distinct from those regulated by vvl protein. Two different isoforms of acj6 protein appear to exert complementary functions, simultaneously activating and inhibiting two distinct sets of transcription units.

acj6 protein cannot bind DNA due to the lack of two basic amino acids in the N-terminal region of its POU homeodomain that are present in other POU doman genes. Immunoprecipitation and crosslinking studies show that acj6 protein associates with vvl protein in vivo. When bound by acj6 protein as a heterodimer, vvl protein no longer binds the enhancer site of the Ddc gene. Transfection experiments in CV-1 cells demonstrate that vvl protein activates transcription of a reporter gene driven by the Ddc promoter or a heterologous promoter preceded by the vvl element. This activation does not occur in the presence of acj6 protein.