Amino terminal tyrosine phosphorylation of human MIXL1

Seven members of the Mix family of paired-type homeoproteins regulate mesoderm/endoderm differentiation in amphibians. In mammals, the MIXL1 (Mix. 1 homeobox [Xenopus laevis]-like gene 1) gene is the sole representative of this family. Unlike the amphibian Mix genes that encode an open reading frame of >300 amino acids, mammalian MIXL1 encodes a smaller protein (~230aa). However, mammalian MIXL1 contains a unique proline-rich domain (PRD) with a potential to interact with signal transducing Src homolgy 3 (SH3) domains. Notably, human MIXL1 also contains a unique tyrosine residue Tyr20 that is amino-terminal to the PRD. Here we report that mammalian MIXL1 protein is phosphorylated at Tyr20 and the phosphorylation is dramatically reduced in the absence of PRD. Our findings are consistent with Tyr20 phosphorylation of MIXL1 being a potential regulatory mechanism that governs its activity.