Apobecl edits the ApoB mRNA by deaminating nucleotide C-6666, which results in a codon change from Glutamate to stop, and subsequent expression of a truncated protein. Apobecl is regulated by ACF (Apobecl complementation factor) and hnRNPQ, which contains an N-terminal "acidic domain" (AcD) of unknown function, three RNA recognition motifs, and an Arg/Gly-rich region. Here, we modeled the structure of AcD using the bacterial protein Barstar as a template. Furthermore, we demonstrated by in vitro pull-down assays that 6xHis-AcD alone is able to interact with GST-Apobecl. Finally, we performed in silico phosphorylation of AcD and molecular dynamics studies, which indicate conformational changes in the phosphorylated form. The results of the latter studies were confirmed by in vitro phosphorylation of 6xHis-AcD by protein kinase C, mass spectrometry, and spectroscopic analyses. Our data suggest hnRNPQ interactions via its AcD with Apobecl and that this interaction is regulated by the AcD phosphorylation. (c) 2006 Elsevier Inc. All rights reserved.