The interaction between SH2 domains and phosphotyrosine residues is central to many signalling pathways in metazoans, including those that involve STATs(signal transducers and activators of transcription). Dictyosteliumis the only non-metazoan known to have functional SH2 domains and Zhukovskaya et al. now describe the SH2 domain protein Dd-STATb, the third known Dictyostelium STAT (see ). Although the SH2 domain of Dd-STATb is highly divergent from other SH2 domains, Dd-STATb is biologically functional: Dd-STATb-null cells have a slight growth disadvantage when co-cultured with parental cells, and gene expression patterns are altered in Dd-STATb- cells when compared with Dd-STATb+ cells. Like other STATS,Dd-STATb exists as a homodimer. But, say the researchers, homodimerisation of Dd-STAT is unlikely to occur through the orthodox SH2 domain:phosphotyrosine interaction seen in animal STATs, and so may represent a non-canonical signalling mechanism not seen in animals.
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