Flagellar localization of taurocyamine kinase 2 in brown algal swarmers

Flagellar motility requires continuous ATP supply; however, the molecular systems supporting ATP buffering in brown algal flagella remain largely unknown. In this study, we examined the intracellular localization of taurocyamine kinase 2 (TK2), a phosphagen kinase implicated in ATP regeneration. Immunofluorescence microscopy showed that TK2 localizes to the flagella of Ectocarpus sp. 7 male gametes and is present on the developing flagella within the plurilocular gametangia prior to gamete release. TK2 was also detected in the flagella of Saccharina japonica zoospores and sperm, indicating a conserved flagellar association with different reproductive modes and phototactic behaviours across species. Although the subcellular localization of TK1 remains unclear, a mitochondrial targeting signal peptide has been identified in many brown algal species. These findings suggest that TK2 is a conserved flagellar component in brown algae that contributes to basic flagellar motility by supporting localized ATP regeneration.