AP2IV3

AP2IV-3 is a nuclear ApiAP2 transcription factor in Toxoplasma gondii that acts as a transcriptional activator driving bradyzoite (tissue cyst) development (PMID:28251183). Deletion of AP2IV-3 reduces tissue cyst formation while conditional overexpression increases it, and the factor directly binds and activates the BAG1 promoter, functionally opposing the repressor AP2IX-9 on a shared set of bradyzoite target genes (PMID:28251183). AP2IV-3 protein is induced during alkaline-stress-driven differentiation, peaking after AP2IX-9 has begun to decline, placing it temporally downstream in the differentiation cascade (PMID:28251183). In dividing tachyzoites its mRNA is cell-cycle-regulated, peaking during the S/M phase (PMID:20865045). Beyond this activator role in bradyzoite gene expression, no additional mechanistic detail has been characterized in the available corpus.

1. 2010 Medium

   Before its function was known, AP2IV-3 was placed within the periodic regulatory program of the tachyzoite cell cycle, establishing it as one of a series of AP2 factors with tightly phased expression.

   Evidence Genome-wide cell cycle transcriptome profiling (microarray) of synchronized tachyzoites

   PMID:20865045

   Open questions at the time

   • mRNA periodicity does not establish protein-level dynamics or transcriptional targets
   • no functional role assigned at this stage
   • does not connect cell-cycle expression to bradyzoite differentiation
2. 2017 High

   The question of what AP2IV-3 does was answered by showing it is a transcriptional activator of bradyzoite development that directly regulates the BAG1 promoter and antagonizes the repressor AP2IX-9.

   Evidence Gene knockout, conditional overexpression (FKBP/Shield-1), BAG1 promoter-luciferase reporter, chromatin immunoprecipitation, and transcriptome profiling in T. gondii

   PMID:28251183

   Open questions at the time

   • genome-wide direct binding sites beyond BAG1 not mapped
   • the biochemical basis of antagonism with AP2IX-9 (competition, co-occupancy, or mutual exclusion) not resolved
   • no structural or DNA-motif characterization of AP2IV-3 binding
3. 2017 Medium

   The temporal placement of AP2IV-3 in the differentiation cascade was defined by showing its protein induction follows the decline of AP2IX-9 during alkaline stress.

   Evidence Immunofluorescence and western blot time-course during alkaline-stress-induced differentiation

   PMID:28251183

   Open questions at the time

   • upstream signals that induce AP2IV-3 expression not identified
   • temporal ordering shown in a single differentiation model (alkaline stress)

• How AP2IV-3 cell-cycle-regulated expression in tachyzoites relates mechanistically to its bradyzoite activator function, and its full genome-wide target repertoire, remain open.
• no genome-wide occupancy map for AP2IV-3
• molecular mechanism of AP2IV-3/AP2IX-9 antagonism unresolved
• connection between S/M-phase expression and differentiation role unexplained