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The Plasmodium helical interspersed subtelomeric proteins or ring-infected erythrocyte surface antigens are a family of protein domains found in the malaria-causing Plasmodium species. It was initially identified as a short four-helical conserved region in the single-domain export proteins, but the identification of this part associated with a DnaJ domain in P. falciparum RESA has led to its reclassification as the RESA N-terminal domain. This domain has been classified into three subfamilies, PHISTa, PHISTb, and PHISTc.
The PHIST proteins are exported to the cytoplasm of the infected erythrocyte. The human malaria parasites P. falciparum and P. vivax have shown a lineage-specific expansion of proteins with this domain. Of the two PHIST genes in the mouse parasite P. berghei, only one is required for infection. The PHIST domain folds into three long helices and two smaller N-terminal helices, and is monomeric in solution. It binds PfEMP1 ATS C-terminus and plays a role in "knob" formation.