Bioinformatic Analyses of the Ataxin-2 Family Since Algae Emphasize Its Small Isoforms, Large Chimerisms, and the Importance of Human Exon 1B as Target of Therapies to Prevent Neurodegeneration

Polyglutamine expansion in Ataxin-2 (ATXN2) is responsible for rare, dominantly in-herited Spinocerebellar Ataxia type 2 (SCA2). Together with its paralog Ataxin-2-like (ATXN2L), both proteins received much interest since deletion of their yeast and fly orthologs alleviates TDP-43-triggered neurotoxicity in Amyotrophic Lateral Sclerosis models. Their typical structure across evolution combines LSm with LSm-Associated Domains and a PAM2 motif. To understand the physiological regulation and functions of Ataxin-2 homologs, the phylogenesis of sequences was analyzed. Human ATXN2 harbors multiple alternative start codons, e.g. from an intrinsically disordered se-quence (IDR) present since armadillo, or from the polyQ sequence that arose since amphibians, or from the LSm domain since primitive eukaryotes. Multiple smaller isoforms also exist across the C-terminus. Therapeutic knockdown of polyQ expansions in human ATXN2 should selectively target exon 1B. PolyQ repeats developed repeat-edly, usually framed and often interrupted by (poly)Pro, originally near PAM2. The LSmAD sequence appeared in algae as the characteristic Ataxin-2 feature with strong conservation. Frequently, Ataxin-2 has added domains, likely due to transcriptional readthrough of neighbor genes during cell stress. These chimerisms show enrichment of rRNA processing; nutrient store mobilization; membrane strengthening via lipid, protein, and glycosylated components; and cell protrusions. Thus, any mutation of Ataxin-2 has complex effects, also affecting membrane resilience.