Tyrosine Phosphorylation in Dictyostelium
Phosphotyrosine signaling in Dictyostelium
Dictyostelium has extensive pTyr-based signaling, despite the lack of TK-group tyrosine kinases. Conservation of some components and pathways with those of humans suggests that pTyr-based signaling preceded animal multicellularity and the invention of TKs, and was originally mediated by dual-specificity TKL kinases that were later superceded by TKs.
Dictyostelium has a large group of TLK kinases, most of which do not fall into known metazoan groups. x of the 66 TKLs are receptor kinases, and six TKLs are known to phosphorylate tyrosine (ZakA and Dypk2-4 phosphorylate tyrosine and SplA and Shk1 are dual specificity (Goldberg et al., 2006 and references therein)). Four other TKLs (Shk2-4) are fused to SH2 domains, and are paralogs of the Shk1 dual-specificity kinase, suggesting that these may also phosphorylate tyrosine.
A more recent report indicates that rk3 (VSK3) is a tyrosine specific receptor kinase, based on in vitro peptide activity, though it is expressed on internal vesicles rather than the plasma membrane (Fang et al JCB 178:411-423). This suggests that its paralogs, rk1-2, may also have TK activity. All three have a W in place of the Y in the YmAPE motif. Y is highly conserved across ser/thr kinases, but is almost always a W in TK-group kinases. The activation loop in this family is HRDLKSHN, more typical of Ser/Thr kinases. Shk1 and Shk4 also show this Y to W change, as do several other TKL kinase in Dictyostelium. None of these kinases have an obvious phosphorylable tyrosine in their activation loops.
In addition to these 5 TKLs, another 7 Dictyostelium proteins have SH2 domains, including 4 STAT proteins, a c-Cbl ortholog (both are orthologs of the human genes that are regulated by tyrosine phosphorylation) and two other proteins (an LRR protein, Lrrb, and an FbxB, an F-box and ankyrin repeat containing protein; neither is homologous to any other published SH2-containing protein). Two of the STATs is known to be tyrosine phosphorylated, and one conserves the tyrosine residue known in human to be a phosphosite.
The Shk1 kinase is membrane-localized, dependent on it's SH2 domain, suggesting the presence of membrane-associated phosphotyrosine and receptor tyrosine kinases, and maybe reminiscent of Src and other receptor-associated kinases in metazoans.
GSKA in Dictyostelium is regulated by ZakA through tyrosine phosphorylation on the activation loop Tyr214 and probably Tyr220 (the equivalent of the autophosphorylation site Tyr216 in human GSK3bs). This phosphorylation is driven by the presence of extracellular cAMP hormone. GSK3 in turn phosphorylates STATa, driving its nuclear export (Ginger et al EMBO J. 19:5483-5491). Another STAT, STATc, is activated by tyrosine phosphorylation during development, and dephosphorylated by the PTP, PTP3. That dephosphorylation is in turn blocked by the fucntion of CblA, the c-Cbl homolog (Langenick et al, J. Cell Science, 121:3524-30) , thus tying several Ptyr-associated proteins together in developmental signaling cascades.
The tyrosine phosphorylation of GSK3 and STAT, the presence of SH2 domains and their association with putative tyrosine- or dual-specificity kinases and the involvement of Cbl in Ptyr pathways are all conserved from Dictyostelium to human, suggesting that eukaryotic tyrosine phosphorylation pathways emerged using TKLs, which were later replaced by TKs in metazoans.
Dictyostelium has 4 PTP phosphatases and members of several other phosphatase families, and does not have PTB pTyr-binding domains.
