Phototropin localization and interactions regulates photophysiological
processes inChlamydomonas reinhardtii
Authors: Sharma Sunita , Sushmita Kumari , Singh Rajani , Sanyal Sibaji K. , Kateriya Suneel 
Published: January 2025

Abstract
Phototropin, a blue-light sensing serine/threonine kinase, plays a pivotal
role in regulating diverse photophysiological processes in both plants and
algae. InChlamydomonas reinhardtii, phototropin
(CrPhot) localizes to the eyespot and flagella, coordinating key cellular
functions such as phototaxis, photosynthesis, gametogenesis, and chlorophyll
biosynthesis. While previous research has identified phototropin interactions
with signaling proteins such as channelrhodopsins and light-harvesting complex
proteins, many aspects of its interaction network and regulatory mechanisms
remain unresolved. In this study, we explored novel interacting protein partners
of phototropin and their roles in modulating its regulatory functions
inChlamydomonas reinhardtii. Employing a suite of
intraflagellar transport (IFT) mutants ofC.
reinhardtiisuch as IFT172, IFT52, IFT88, IFT139,
kinesin/dynein, CEP290 etc., we elucidate that phototropin localization within
the flagella and eyespot is IFT-mediated. Our study highlights interaction of
phototropin with other photoreceptors-channelrhodopsins (ChR1 and ChR2),
chlamyopsin 6, LOV-histidine kinases (LOV-HK1, LOV-HK2) and signaling
protein-14-3-3. CRISPR-Cas9 knockouts of phototropin showed reduced ChR1, 14-
3-3 levels and exhibited impaired photomotility. Moreover, two LOV-domain
containing histidine kinases, LOV-HK1 and LOV-HK2, were identified
inC. reinhardtii. Gene expression of LOV-HK1 and
LOV-HK2 were found to be elevated in UV-light inC.
reinhardtiiand their genes expression was found to be altered
in phototropin CRISPR-Cas9 knockouts. This study provides new insights into
phototropin signalosome and highlights molecular mechanisms governing its
function. The research outcomes advances our understanding of phototropin
trafficking and signal modulation inChlamydomonas
reinhardtii, and sets the stage for further exploration into
the broader physiological roles of phototropin in cellular responses.

Graphical abstract
Phototropin, a blue-light receptor inChlamydomonas
reinhardtii, localizes to the flagella and eyespot,
mediates phototaxis and photosynthesis. Its trafficking is mediated by
intraflagellar transport (IFT) machinery, with mutations in IFT components
(kinesin, dynein, IFT172, IFT52, IFT88, IFT139, CEP290) disrupting
phototropin localization. Phototropin interacts with other photoreceptors
like channelrhodospins (ChR1/2), chlamyopsin 6, LOV-histidine kinases
(LOV-HK1, LOV-HK2) and signaling proteins (14-3-3), coordinating
light-driven responses. These findings underscore the details of phototropin
trafficking and phototropin signaling impacting light-induced physiological
processes inC. reinhardtii.

Highlights
• Phototropin localizes in eyepot and flagella
inChlamydomonas reinhardtii.
• Intraflagellar transport (IFT) mutants ofC.
reinhardtiisuggest role of different IFT proteins in
phototropin trafficking and localization.
• Phototropin interacts with other photoreceptors (ChR1 & ChR2,
COP6, LOV-HK1 & LOV-HK2) and signaling proteins (14-3-3), contributing
to various physiological processes.
• CRISPR-Cas9 knockouts of phototropin showed reduced 14-3-3 protein
content and photomotility response inC.
reinhardtii.