passages.id: split_4_test_715_passage
passages.type: progene_text
passages.text: In heterodimers , a conserved tryptophan motif in Hox proteins binds a pocket on the surface of the Pbx homeodomain , while Meis / Prep1 proteins bind an N-terminal Pbx domain , raising the possibility that the tryptophan - interaction pocket of the Pbx component of a Pbx - Meis / Prep1 complex is still available to bind trypto - phan motifs of other transcription factors bound to flanking elements .