NAPL

NAPL (PF3D7_1203700) is an essential, dimeric histone chaperone of Plasmodium falciparum that binds histones in the cytoplasm but does not itself deposit them onto DNA, marking it as a nucleocytoplasmic histone relay rather than a direct chromatin assembly factor (PMID:16638744, PMID:19176479). It exists as a dimer in solution and preferentially binds the H3-H4 tetramer over H2A-H2B, and also engages linker histone H1 (PMID:15899528, PMID:16638744). NAPL is phosphorylated by parasite casein kinase II in a heparin-inhibitable manner, and this phosphorylation increases its affinity for H3, H4, and H1 approximately three-fold (PMID:15899528, PMID:16638744). Its histone cargo can be extracted by the nuclear paralog PfNAPS, consistent with a hand-off role in which NAPL delivers histones toward chromatin without depositing them (PMID:16638744). The 2.3 Å crystal structure reveals a dimeric NAP-fold comprising a dimerization helix (domain I) and an α-helical/β-subdomain (domain II) but lacks the accessory domain found in yeast NAP-1 (PMID:19176479, PMID:19465776). Consistent with this shuttling role, NAPL localizes exclusively to the cytoplasm across asexual and sexual erythrocytic stages and does not co-localize with PfNAPS, and gene-disruption attempts failed, establishing it as essential for parasite viability (PMID:15899528, PMID:19176479). Histone-recognition mapping indicates NAPL contacts a surface-exposed patch on the histone octamer that overlaps the DNA-wrapping path, and this interaction is tuned by histone post-translational modifications (PMID:22771717).

1. 2005 Medium

   Establishing that PF3D7_1203700 is a histone-binding protein defined its candidacy as a chaperone and distinguished it from its nuclear partner by localization.

   Evidence ELISA histone-binding assays, biophysical dimer characterization, in-cell phosphorylation with heparin inhibition, and immunofluorescence across erythrocytic stages in P. falciparum

   PMID:15899528

   Open questions at the time

   • ELISA binding does not establish physiological affinity or stoichiometry
   • casein kinase II involvement inferred from heparin inhibition, not direct identification
   • cytoplasmic localization from immunofluorescence not corroborated by fractionation
2. 2006 High

   Defining NAPL as unable to deposit histones onto DNA yet capable of handing them to PfNAPS reframed it from an assembly factor to a nucleocytoplasmic histone relay, and linked its function to CKII regulation.

   Evidence In vitro histone deposition assays, in vivo/in vitro CKII phosphorylation with heparin controls, and histone-binding affinity measurements in P. falciparum

   PMID:16638744

   Open questions at the time

   • the in vivo directionality of the NAPL-to-PfNAPS hand-off was not demonstrated in cells
   • the CKII phosphosites on NAPL and their in vivo occupancy were not mapped
   • functional consequence of the ~3-fold affinity increase for parasite chromatin dynamics untested
3. 2009 High

   Solving the NAPL crystal structure and demonstrating failed gene disruption provided the atomic architecture of a NAP-fold and established the protein as essential.

   Evidence X-ray crystallography at 2.3 Å (iodide-SAD/SIRAS phasing), GFP-fusion live microscopy, and gene-disruption attempts in P. falciparum

   PMID:19176479 PMID:19465776

   Open questions at the time

   • the functional significance of the missing accessory domain not tested by mutagenesis
   • no histone-bound structure to define the interaction interface
   • essentiality inferred from failed disruption rather than conditional knockdown
4. 2012 Medium

   Mapping the histone-recognition surface showed NAPL contacts an octamer patch overlapping the DNA path and is modulated by histone PTMs, providing a structural rationale for mutually exclusive NAPL/DNA binding.

   Evidence Histone peptide interaction arrays and NMR chemical shift perturbation in P. falciparum

   PMID:22771717

   Open questions at the time

   • findings reported for Nap generally (both PfNapL and PfNapS), so attribution to NAPL specifically is partial
   • peptide arrays may not recapitulate intact octamer binding
   • the in vivo relevance of the PTM-tuned binding untested

• How NAPL's histone cargo is directed toward the nucleus and handed to PfNAPS in vivo, and how CKII phosphorylation is regulated across the parasite life cycle, remain unresolved.
• no in vivo demonstration of the proposed nucleocytoplasmic shuttling
• CKII phosphosites and their dynamic regulation unmapped
• no conditional depletion to define essential function