The protein Hrb57A of Drosophila melanogaster closely related to hnRNP K from vertebrates is present at sites active in transcription and coprecipitates with four RNA-binding proteins

The hnRNP K protein is among the major hnRNA-binding proteins with a Strong preference for cytidine-rich sequences. We have cloned a Drosophila hnRNP protein closely related to this vertebrate protein, The protein first ident ified by the monoclonal antibody Q18 is encoded by a gene located in 57A on polytene chromosomes and has been consequently named Hrb57A. The amino aci d sequence of the Hrb57A KH domains and their overall organisation in the p rotein are remarkably similar to the vertebrate proteins. As the hnRNP K in vertebrates the M-r 55 000 Drosophila Hrb57A/Q18 protein strongly binds to poly(C) in vitro and is ubiquitously present in nuclei active in transcrip tion. On polytene chromosomes it is found in many puffs and minipuffs. Hrb5 7A/Q18 specifically coprecipitates four other proteins: Hrb87F/P11 a Drosop hila hnRNP Al homologue, the hnRNA-binding protein S5, the RNA recognition motif-containing protein NonA and the RNA-binding zinc finger-containing pr otein on ecdysone puffs PEP/X4, (C) 2000 Elsevier Science B.V. All rights r eserved.