Nucleosome Assembly Protein 1 in Dinoflagellate Cells Without Architectural Nucleosomes

Nucleosome assembly protein 1 (NAP1) is highly conserved across eukaryotes, yet its biochemical function, particularly its proposed role as a histone chaperone, remains unresolved. Dinoflagellates, including Karenia brevis (Kb) and Crypthecodinium cohnii (Cc), lack architectural nucleosomes and express core histones at unusually low levels, yet retain abundant NAP1 transcripts encoding two to three distinct homologs. Confocal immunolocalization of K. brevis showed strong nuclear signals for both homologs, between chromosomes and at the nucleolus, with higher nuclear-to-cytoplasmic ratios in G₂ than in G₁ (the two gap phases of the cell cycle); labeling at chromosome-territory margins and at the nuclear cortex is consistent with an association with the telomeric nucleosomes anchored to the nuclear envelope. Recombinant KbNAP1Bp reproduced the canonical yeast NAP1 fold, recovered H2A-immunoreactive material in immunoprecipitation, and preferentially retarded larger DNA fragments in gel mobility assays, whereas KbNAP1Ap did none of these under the conditions tested. In C. cohnii, the NAP1 protein peaked at S-G₂, and exposure to a CcNAP1.1-antisense oligodeoxynucleotide (ODN) was associated with an S-G₂/M delay. These findings uncouple NAP1 abundance from the availability of a canonical nucleosomal substrate and suggest evolutionary repurposing toward non-nucleosomal roles in chromosome-territory organization. They help define the minimal functional core of this conserved chaperone family and caution against treating NAP1 abundance as a proxy for nucleosome assembly activity.