RAPID, TIME-RESOLVED PROXIMITY LABELING BY SBP1 IDENTIFIES A PORIN DOMAIN PROTEIN AT THE MALARIA PARASITE PERIPHERY

Abstract

The deadly human malaria-causing parasite, Plasmodium falciparum relies on its capacity to completely remodel its host red blood cell (RBC) through the export of hundreds of parasite proteins across several membranes to the RBC. Among these exported proteins are numerous membrane proteins that are inserted into the parasite plasma membrane (PPM) during their transport via the secretory pathway. It is not known how these exported membrane proteins are extracted from the PPM for export. To answer this question, we fused the exported membrane protein skeleton binding protein 1 (SBP1) with the rapid, efficient, and promiscuous biotin ligase known as TurboID (SBP1TbID). Our data show that the SBP1TbID fusion protein was exported efficiently to the host RBC and was able to rapidly biotinylate proteins at the host-parasite interface during its export as well as at its final destination in the host RBC. Using time-resolved proximity biotinylation and label-free quantitative proteomics, we identified early (pre-export) interactors and late (post-export) interactors of SBP1TbID. This led to the identification of 24 proteins that were 10-fold or more enriched in the pre-export time point compared to the post-export time point. Among these early interactors were two promising membrane-associated proteins, one of which has a predicted porin domain, that could potentially act as a translocon at the PPM for exported membrane proteins (Plasmodium translocon of exported membrane proteins or PTEM). Both proteins localize to the host-parasite interface during early stages of the intraerythrocytic cycle and conditional knockdown of these candidates show that they play essential roles in the asexual lifecycle of the parasite. Taken together, our data suggest that these two proteins may play a role in extracting membrane proteins from the PPM for export to the host RBC.