Mechanistic Constraints on ClpM Expression Underlie Apicoplast Genome Retention in Malaria Parasites
The apicoplast of malaria parasites retains a reduced genome encoding a small set of genes with unknown functions. Among these genes is a putative ClpM chaperone, which unlike other apicoplast Clp-family members is not nuclear but plastid-encoded. In this study, we used ClpM as a model case to investigate evolutionary and molecular basis for plastid genome retention. Phylogenetic analyses across plastid-containing eukaryotes revealed that ClpM orthologues are broadly conserved and consistently plastid-encoded in all organisms with a red alga-derived plastid, irrespective of parasitism, photosynthesis or physiology. This broad phenomenon suggested gene-specific evolutionary constraints that were subsequently tested experimentally. To test whether clpM can be functionally expressed from the nucleus, we generated transgenic parasites carrying a nuclear ClpM copy fused to an apicoplast-targeting transit peptide. Unexpectedly, standard transgenesis resulted in transcriptional silencing, and we therefore forced transcription using integration into an endogenous essential locus. This led to robust clpM mRNA, however no detectable ClpM protein was observed. Multiple analyses ruled out apicoplast-dependent instability, ER-associated degradation, misfolding or membrane sequestration. Attempts to express clpM or other plastid-derived genes using endogenous sequences were found to be toxic, suggesting nucleotide-sequence incompatibility. In contrast, a transgene carrying a second copy of the nuclear ClpC ortholog was readily expressed. Comparative analysis of ClpM and ClpC domain architecture showed that their ATPase domains form distinct evolutionary clusters, suggesting conserved but functionally divergent roles. Subsequently, domain-swap experiments between ClpC and ClpM rescued partial expression and identified specific domains as contributors to the nuclear-expression barrier. Together, these findings demonstrate that clpM retention in the apicoplast genome is enforced by multilayered constraints involving evolutionary conservation, nucleotide-sequence incompatibility, transcriptional block and protein-intrinsic translational barriers. This work provides experimental evidence for mechanisms that restrict organelle-to-nucleus gene transfer and contribute to organelle genome retention.