AUF1/hnRNPD is an ALK partner. (A) ALK-associated proteins were immunoprecipitated from ALK-positive SU-DHL1 and control FEPD cell lines using anti-ALK (ALK1) antibody. The band labeled AUF1, whose intensity was clearly different in NPM-ALK-positive and -negative protein extracts, was identified by proteomic and mass spectrometry analyses as p45^AUF1. (B) Peptide sequence of AUF1. Bold type indicates the peptides used to identify AUF1 (31% recovery). The 2 alternative exons (2 and 7) are boxed. Seven tyrosine residues, common to all 4 isoforms, are found in the core of AUF1, and 14 are located in the alternative carboxy-terminal sequence encoded by exon 7, found in p42^AUF1 and p45^AUF1 isoforms. Except for tyrosine 244 (numbered according to the human sequence), found only in rat, mouse, and human AUF1 proteins, all these tyrosine residues are present in chicken and Xenopus AUF1 proteins. The conserved tyrosine residues are underlined and those predicted to be efficiently phosphorylated³¹  (score > 0.6) are encircled. (C) AUF1 protein contains 2 RNA recognition motifs (RRM1, RRM2) and comprises 4 isoforms, p37^AUF1, p40^AUF1, p42^AUF1, and p45^AUF1, depending on the presence or absence of the 19-amino acid N-terminal and/or 49-amino acid C-terminal inserts.