# Two Centrins and Their Posttranslational Modification Modulate the Cell Cycle of Giardia lamblia

## TL;DR

This study shows that two centrins in Giardia lamblia are essential for cell division and shape, and their function is regulated by a chemical modification called SUMOylation.

## Contribution

The study identifies SUMOylation as a novel regulatory mechanism for Giardia centrins during cell cycle progression and morphogenesis.

## Key findings

- Knockdown of Glcents causes defects in nuclear positioning and cytokinesis in Giardia.
- Glcents are substrates of SUMO, and SUMOylation inhibition disrupts their localization and cell cycle progression.
- Blocking SUMOylation leads to conformational changes in Giardia, including altered ventral disc shape and peripheral vesicles.

## Abstract

Centrins, Ca2+‐binding proteins conserved in eukaryotes, are the key components of the microtubule‐organizing center. Giardia lamblia possesses two centrins (GL50803_6744: centrin 1; GL50803_104685: centrin 2) localized in the basal bodies during cell division. G. lamblia centrin 2 (Glcent2) is also found in the nuclei of interphase Giardia, with its N‐terminal half being necessary for this localization. Morpholino‐mediated knockdown of Glcents resulted in abnormal nuclear positioning and cytokinesis, causing cell malformations, including ventral discs and flagella defects. Small ubiquitin‐like modifier (SUMO)ylation is a posttranslational modification, which modulates several cellular processes. Here, we demonstrated that Glcents are substrates of SUMO through in vitro SUMOylation and immunoprecipitation experiments. Additionally, treatment of Giardia with ginkgolic acid, which inhibits the E1 enzyme of the SUMO pathway, and CRISPRi‐mediated inhibition of G. lamblia Ubc9, the E2 conjugation enzyme involved in SUMOylation, resulted in defects in the localization of Glcents. Blocking SUMOylation resulted in the arrest of Giardia cells and conformational changes, including alterations in the ventral disc shape, posterolateral flanges, and peripheral vesicles. Taken together, we demonstrated that Glcents function in Giardia cell cycle progression and morphogenesis, with the activity of both Glcents being modulated by SUMOylation.

Two Giardia lamblia centrins (Glcents) play a role in cell cycle progression and morphogenesis of Giardia. Both Glcents are substrates of GlSUMO, suggesting a role of SUMOylation in their proper localization. SUMOylation is also involved in regulating vacuole formation in Giardia.