Apicortin, a Putative Apicomplexan-Specific Protein, Is Present in Deep-Branching Opisthokonts

Area of Science:

• * Evolutionary biology
• * Molecular biology
• * Genomics

Background:

• * Apicortin, a tubulin/microtubule-binding protein, was initially identified in Apicomplexa.
• * Its presence was previously noted in *Trichoplax adhaerens* (Placozoa) and zoosporic fungi.
• * Apicortin is found across diverse Apicomplexa taxa, including Myzozoa (Chrompodellids, Squirmids, Dinoflagellates, Perkinsids).

Purpose of the Study:

• * To investigate the broader evolutionary distribution of apicortin beyond Apicomplexa.
• * To identify additional opisthokont lineages harboring apicortin-homologous sequences.
• * To clarify the presence and origin of apicortin in various animal phyla.

Main Methods:

• * Bioinformatic analysis of sequenced genomes and transcriptomes.
• * Comparative genomics to identify apicortin homologs.
• * Phylogenetic analysis to determine evolutionary relationships.

Main Results:

• * Apicortin is widely distributed in deep-branching opisthokonts, including fungi, Placozoa, Ctenophora, and choanoflagellates.
• * Apicortin is confirmed in multiple Placozoa species and Ctenophora.
• * Apicortin-homologous sequences in bilaterian animals were identified as contamination.

Conclusions:

• * Apicortin is an ancient protein with a broad distribution in early-branching opisthokonts.
• * Its presence in Placozoa, Ctenophora, and choanoflagellates provides insights into early animal evolution.
• * The absence of genuine apicortin in bilaterians suggests its loss or divergence in these lineages.