ClpR

ClpR (PF3D7_1436800) is a catalytically inactive paralog of the ClpP protease subunit that localizes to the apicoplast of blood-stage P. falciparum (PMID:20887733, PMID:23192353). Like PfClpP, ClpR assembles predominantly into homoheptameric rings, as shown by size-exclusion chromatography, analytical ultracentrifugation, and electron microscopy (PMID:20887733). The X-ray crystal structure shows that the ClpR monomer adopts a ClpP-like fold but carries a unique 'R-motif' β-turn near an altered, non-functional catalytic triad, and its heptamer forms a more open, flat ring than a ClpP heptamer (PMID:23192353). In contrast to Clp orthologs in other organisms, ClpR and PfClpP do not form a stable heterocomplex in the P. falciparum apicoplast (PMID:23192353), though the original characterization proposed the existence of a ClpCRP complex (PMID:20887733). Beyond its structural characterization, localization, and non-functional catalytic status, no substrate, enzymatic activity, or defined complex role for ClpR has been characterized in the available corpus.

1. 2010 Medium

   Establishing whether P. falciparum encodes a Clp proteolytic component and where it acts required identifying ClpR as an inactive ClpP paralog that forms homoheptameric rings and resides in the apicoplast.

   Evidence Size-exclusion chromatography, analytical ultracentrifugation, and electron microscopy of PfClpR, with subcellular localization in blood-stage parasites

   PMID:20887733

   Open questions at the time

   • The proposed ClpCRP complex was inferred, not directly reconstituted
   • No substrate or physiological function of ClpR was defined
   • Whether ClpR contributes any regulatory or scaffolding role was not addressed
2. 2012 High

   Determining the structural basis of ClpR's catalytic inactivity and its interaction behavior showed a ClpP-like fold with a unique R-motif near a non-functional triad and, unexpectedly, no stable heterocomplex with PfClpP.

   Evidence X-ray crystallography of the PfClpR heptamer plus biochemical interaction assays and apicoplast localization

   PMID:23192353

   Open questions at the time

   • The functional purpose of the open, flat heptameric ring is unknown
   • Absence of a stable PfClpP-PfClpR heterocomplex leaves ClpR's operational partner undefined
   • No proteolytic substrate or in vivo role established

• What functional role ClpR plays given its catalytic inactivity and its failure to form a stable complex with PfClpP remains unresolved.
• No physiological substrate or binding partner for ClpR has been identified
• The in vivo consequence of ClpR loss or the essentiality of ClpR has not been characterized
• How the R-motif and open ring architecture contribute to apicoplast Clp machinery function is unknown