The Regulation of TCF POP-1 Asymmetry and its Function in Early Cell Fate Specification in Caenorhabditis Elegans
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Cell-cell signaling pervades all aspects of development. Signaling pathways need to be highly robust to ensure reproducible outcomes. In the cases of several major pathways, including the Wnt signaling pathway, default repression and signal-dependent activation are both mediated by the same response elements and transcription factors. My work focuses on how the C. elegans TCF protein POP-1 is converted from a repressor into an activator upon Wnt and MAPK signaling. C. elegans embryos exhibit a nearly invariant cell lineage composed primarily of a stepwise binary diversification of anteroposterior blastomere identities. The nuclear level of TCF/POP-1 is lowered in all posterior cells in the A-P cell divisions. I showed that the ?-catenin homolog, SYS-1, exhibits reiterated asymmetry which is reciprocal to that of POP-1. SYS-1 functions as a limiting coactivator for POP-1. The SYS-1-to-POP-1 ratio is critical for both anterior and posterior cell fates. A high ratio drives the posterior cell fate whereas a low ratio drives the anterior cell fate in multiple A-P divisions throughout development. I showed genetically that while POP-1 nuclear asymmetry is mainly regulated by the MAPK pathway, SYS-1 asymmetry is regulated solely by the Wnt pathway. Besides the quantitative differences between A-P nuclei, the Wnt and MAPK pathways also modify POP-1 qualitatively, affecting the transcriptional activity of POP-1. I showed that LIT-1/WRM-1-dependent phosphorylation of POP-1 reduces its binding to the coactivator SYS-1. Deleting the PCA domain also weakens the interaction between POP-1 and SYS-1 independent of LIT-1/WRM-1 phosphorylation. The PCA domain is required for POP-1 nuclear asymmetry and all LIT-1/WRM-1-dependent phosphorylation of POP-1. Within the PCA domain, mutating POP-1 threonine 426 to aspartic acid, but not alanine, shows similar effects to deleting the PCA domain. This study shows that Wnt and MAPK signaling pathways regulate the TCF protein POP-1 both quantitatively for its nuclear asymmetry, and qualitatively for its transcriptional activity. The two pathways not only change the levels of POP-1 and SYS-1 but also modify the strength of binding, strongly favoring the formation of POP-1/SYS-1 activation complex in the posterior cell, thereby driving A-P differential cell fates.