Derlin-like proteins are essential for the function of Euglena gracilis chloroplasts

Despite the growing interest in basic research on Euglena gracilis and its importance in bioproduction, several key questions regarding its cell biology remain unanswered. One of them concerns the mechanism of protein import into its secondary chloroplasts, which are derived from a green alga. In this study, we have proved the essentiality of 2 rhomboid pseudoproteases, DerL-1 and DerL-2, which are predicted to localize in the membrane of the chloroplast envelope. We demonstrate that these proteins belong to a euglenid-specific subgroup unrelated to other eukaryotic rhomboid pseudoproteases. Silencing these proteins using RNA interference leads to a distinct phenotype marked by culture bleaching and delayed growth. This is accompanied by the loss of typical chloroplast structure and a reduction in the expression of a substantial portion of the chloroplast proteome, especially proteins involved in photosynthesis. We hypothesize that these pseudoproteases are involved in chloroplast protein import by forming a core component of the translocon across the middle envelope membrane. This indicates a remarkable case of molecular convergence, as other eukaryotes utilize rhomboid pseudoproteases-unrelated to euglenid proteins-for protein import into plastids derived from red algae.