Ancient intraflagellar transport machinery controls unique spatial distribution of phototropin in an evolutionary important non-flagellated vegetative stage of terrestrial alga

Abstract

Intraflagellar transport (IFT) is a conserved trafficking system in eukaryotes that moves proteins along microtubules. It is best known for its essential role in building and maintaining cilia and flagella. Intriguingly, several IFT components are still found in organisms that no longer possess flagella, raising important questions about their original functions and how they may have been repurposed during evolution. The filamentous alga Klebsormidium nitens, positioned at the base of the streptophyte lineage, offers a valuable model for exploring this transition. Here, we investigate the IFT machinery in K. nitens and its relationship with the blue-light photoreceptor phototropin. Comparative genomic analyses show that key IFT-A and IFT-B components are retained, despite the complete loss of flagella in vegetative state. Cellular detection and immunofluorescence studies revealed the presence and localisation of IFT components, interestingly, their co-localization with phototropin. Notably, IFT-139 and IFT-20 strongly co-localize with phototropin at plasma membrane-associated regions. Phototropin overlapping localization (plasma membrane associated) with conserved phospho-adaptor protein 14-3-3, pointing to a phosphorylation-dependent signaling network. Unlike in Chlamydomonas reinhardtii, where these proteins localize to flagella, their interaction in K. nitens occurs independently of cilia presence. Together, these results evidenced that IFT components were retained and repurposed early in streptophyte evolution and might support phototropin localization and signalling, revealing an ancestral, non-ciliary role for the IFT system.