Protein control of membrane and organelle dynamics: Insights from the divergent eukaryote Toxoplasma gondii

Abstract

Integral membrane protein complexes control key cellular functions in eukaryotes by defining membrane-bound spaces within organelles and mediating inter-organelles contacts. Despite the critical role of membrane complexes in cell biology, most of our knowledge is from a handful of model systems, primarily yeast and mammals, while a full functional and evolutionary understanding remains incomplete without the perspective from a broad range of divergent organisms. Apicomplexan parasites are single-cell eukaryotes whose survival depends on organelle compartmentalisation and communication. Studies of a model apicomplexan, Toxoplasma gondii, reveal unexpected divergence in the composition and function of complexes previously considered broadly conserved, such as the mitochondrial ATP synthase and the tethers mediating ER-mitochondria membrane contact sites. Thus, Toxoplasma joins the repertoire of divergent model eukaryotes whose research completes our understanding of fundamental cell biology.

Figure 1

. Putative membrane contact sites observed and described in Toxoplasma gondii. A scheme of a single Toxoplasma tachyzoite (left) showing mitochondrion (green), ER (light purple), nucleus (dark purple), and inner membrane complex (IMC) (pink). Contact sites discussed in the main text are highlighted in circles with identity of proposed components shown: (1) ER–mitochondrion; (2) nucleus–mitochondrion; and (3) mitochondrion–IMC. EM images of Toxoplasma (middle/right) showing examples of organelle proximity that provide evidence for the contacts shown in (A), whereby A and B are colour coordinated, and C has no added colours for clarity.