Revisiting the role of Toxoplasma gondii ERK7 in the maintenance and stability of the apical complex

Abstract

Toxoplasma gondii ERK7 is known to contribute to the integrity of the apical complex and to be involved only in the final step of the conoid biogenesis. In the absence of ERK7, mature parasites lose their conoid complex and are unable to glide, invade or egress from host cells. In contrast to a previous report, we show here that depletion of ERK7 phenocopies the depletion of the apical cap proteins AC9 or AC10. The absence of ERK7 leads to the loss of the apical polar ring, the disorganization of the basket of subpellicular microtubules and an impairment in micronemes secretion. Ultra-expansion microscopy (U-ExM) coupled to NHS-Ester staining on intracellular parasites offers an unprecedented level of resolution and highlights the disorganization of the rhoptries as well as the dilated plasma membrane at the apical pole in the absence of ERK7. Comparative proteomics analysis of wild-type and ERK7 or AC9 depleted parasites led to the disappearance of known, predicted, as well as putative novel components of the apical complex. In contrast, the absence of ERK7 led to an accumulation of microneme proteins, resulting from the defect in exocytosis of the organelles. Importance The conoid is an enigmatic, dynamic organelle positioned at the apical tip of the coccidian subgroup of the Apicomplexa, close to the apical polar ring (APR) from which the subpellicular microtubules (SPTMs) emerge and at the site of microneme and rhoptry secretory organelles exocytosis. In Toxoplasma gondii, the conoid protrudes concomitantly to microneme secretion, during egress, motility and invasion. The conditional depletion of the apical cap structural protein AC9 or AC10 leads to a disorganization of SPTMs as well as the loss of APR and conoid that result in microneme secretion defect and block in motility, invasion and egress. We show here that depletion of the kinase ERK7 phenocopies completely AC9 and AC10 mutants. Moreover, the combination of ultrastructure expansion microscopy with an NHS ester staining revealed that ERK7 depleted parasites exhibit a dilated apical plasma membrane and a mis-positioning of the rhoptry organelles.