The protein encoded by var2csa uORF plays an important role in ER stress response

Plasmodium falciparum, the protozoan parasite responsible for the deadliest form of human malaria infects millions each year and causes over half million deaths, primarily of children and pregnant women. The severe pathology of pregnancy associated malaria (PAM) is attributed to the parasite`s ability to modify the infected red blood cells to adhere and sequester in the placenta through the binding of VAR2CSA to chondroitin sulfate A (CSA). As a member of the var gene family, mutually exclusive expression of var2csa is regulated at the level of transcription. However, var2csa contain an additional regulatory element: a unique upstream open reading frame (uORF) that inhibits protein translation when transcriptional activation occurs in a non-pregnant individual. We found that in addition to translational regulation, this uORF encodes for a protein that signals for ER localization. This novel protein is expressed independent of VAR2CSA and located to the ER. We determine the specific sequence essential for its ER localization and confirmed its endogenic function using CRISPR/cas9 manipulation. Replacement of the ER signaling domain with an epitope tag mis-localized the uORF to the cytoplasm and induced ER stress followed by significant changes to the ER morphology. Proteomic analysis of the uORF interactome gave additional indications and possible mechanistic insight for the uORF role in ER maintenance under stress conditions.