The parasite Plasmodium falciparum is responsible for the most virulent form of malaria, killing ~500,000 people annually. The parasite’s virulence arises during the asexual blood-stage, in which the parasite invades red blood cells, remodels the host cell to survive, divide, and ultimately egress from the cell for subsequent cycles of invasion. The mature red blood cell is a unique host cell as it lacks endogenous organelles for the parasite to co-opt. So, the parasite deploys ~500 proteins into the host cell to achieve host-cell remodeling and the trafficking of antigens, critical for host immune evasion. Among these exported proteins, none bear semblance to canonical secretory machinery. Instead, more than a quarter of exported proteins contain extended, often repetitious regions. These regions are predicted to be intrinsically disordered regions (IDRs) and deletion of some of these regions in prior work has shown to have marked effects on both host cell remodeling and antigen trafficking.
We hypothesized that the repetitive, often charged, IDRs of exported-proteins facilitate phase separation which could explain the otherwise enigmatic trafficking of parasite-antigens to the host cell surface. To test this hypothesis, predicted-IDRs from six exported proteins were tested with the OPTOdrop system, in which a photoactivatable domain allows for the quantitation of an IDR’s propensity to form droplets. In preliminary experiments, three of our six constructs formed droplets or condensates upon activation, suggesting that phase separation may be at play in the infected red blood cell environment.
In an environment void of canonical trafficking machinery, phase separation offers a mechanistic explanation for regulated antigen trafficking to the host cell surface. The infected red blood cell environment, without RNA or other polymers that may seed droplet formation, may also serve as an interesting environment to study the nuances of IDRs and their roles in droplet-mediated protein trafficking.