p24-Tango1 interactions ensure ER-Golgi interface stability and efficient transport

Role of Toxoplasma gondii p24δ in Regulating the Transition from Tachyzoite to Bradyzoite Development

Toxoplasma gondii is an obligate intracellular parasite capable of infecting warm-blooded vertebrates, including humans. In its intermediate hosts, T. gondii can transition between two life stages: the rapidly replicating tachyzoite and the quiescent bradyzoite. In Saccharomyces cerevisiae, the p24 protein acts as a cargo receptor, cycling between the ER and Golgi in the early secretory pathway to recruit cargo proteins into nascent vesicles. However, the function of p24 in T. gondii remains undefined. In this study, we identified four p24 proteins in T. gondii, with Tgp24δ specifically localizing to the ER-Golgi system. Loss of p24δ in a type Ι strain (RHΔku80) significantly reduced proliferation and virulence in mice. Transcriptome and proteomic analyses showed that TgΔp24δ tachyzoites expressed high levels of bradyzoite-specific genes, including bag1, ldh2, and bpk1, under standard culture conditions. Additional data indicate that TgΔp24δ tachyzoites can differentiate and form bradyzoites in vitro. This suggests that Tgp24δ is important for the parasite's growth.

The pathways of secretory cargo export at the endoplasmic reticulum

Palade's original model proposed that secretory cargo is transported between stable compartments via vesicles. However, recent findings challenge this view, suggesting that secretory pathway compartments are dynamic, with cargo itself dictating whether transfer occurs via vesicles or through the continuity and maturation of compartmental structures. At the heart of this process is TANGO1, a key component of a molecular machine that works in concert with COPII proteins to construct export routes tailored to the size and quantity of secretory cargo.

The exocyst complex controls multiple events in the pathway of regulated exocytosis

Eukaryotic cells depend on exocytosis to direct intracellularly synthesized material toward the extracellular space or the plasma membrane, so exocytosis constitutes a basic function for cellular homeostasis and communication between cells. The secretory pathway includes biogenesis of secretory granules (SGs), their maturation and fusion with the plasma membrane (exocytosis), resulting in release of SG content to the extracellular space. The larval salivary gland of Drosophila melanogaster is an excellent model for studying exocytosis. This gland synthesizes mucins that are packaged in SGs that sprout from the trans-Golgi network and then undergo a maturation process that involves homotypic fusion, condensation, and acidification. Finally, mature SGs are directed to the apical domain of the plasma membrane with which they fuse, releasing their content into the gland lumen. The exocyst is a hetero-octameric complex that participates in tethering of vesicles to the plasma membrane during constitutive exocytosis. By precise temperature-dependent gradual activation of the Gal4-UAS expression system, we have induced different levels of silencing of exocyst complex subunits, and identified three temporarily distinctive steps of the regulated exocytic pathway where the exocyst is critically required: SG biogenesis, SG maturation, and SG exocytosis. Our results shed light on previously unidentified functions of the exocyst along the exocytic pathway. We propose that the exocyst acts as a general tethering factor in various steps of this cellular process.

p24 family Tango(1) at the endoplasmic reticulum exit site to organize cargo exit

The p24 family of proteins have been regarded as cargo receptors for endoplasmic reticulum (ER) to Golgi transport; however, their precise functions have yet to be revealed. In this issue, Pastor-Pareja and colleagues (https://doi.org/10.1083/jcb.202309045) show that the interaction of these proteins with Tango1 is critical for their localization at the ER exit site (ERES) and efficient transport of secretory proteins in Drosophila.