Abstract
Proper development of T cells depends on lineage-specific regulators controlled transcriptionally and posttranslationally to ensure precise levels at appropriate times. Conditional inactivation of F-box protein Fbw7 in mouse T-cell development resulted in reduced thymic CD4 single-positive (SP) and splenic CD4(+) and CD8(+) cell proportions. Fbw7 deficiency skewed CD8 SP lineage differentiation, which exhibited a higher incidence of apoptosis. Similar perturbations during development of CD8-positive cells were reported with transgenic mice, which enforced GATA3 (T-cell differentiation regulator) expression throughout T-cell development. We observed augmented GATA3 in CD4/CD8 double negative (DN) stage 4, CD4 SP, and CD8 SP lineages in Fbw7-deficient thymocytes. Using overexpressed proteins in cultured cells, we demonstrated that Fbw7 bound to, ubiquitylated, and destabilized GATA3. Two Cdc4 phosphodegron (CPD) candidate sequences, consensus Fbw7 recognition domains, were identified in GATA3, and phosphorylation of Thr-156 in CPD was required for Fbw7-mediated ubiquitylation and degradation. Phosphorylation of GATA3 Thr-156 was detected in mouse thymocytes, and cyclin-dependent kinase 2 (CDK2) was identified as a respondent for phosphorylation at Thr-156. These observations suggest that Fbw7-mediated GATA3 regulation with CDK2-mediated phosphorylation of CPD contributes to the precise differentiation of T-cell lineages.