Abstract
In Chlamydomonas, the directly light-gated, plasma
membrane-localized (PM) cation channels channelrhodopsins ChR1 and ChR2
are the primary photoreceptors for phototaxis. Their targeting and
abundance is essential for optimal movement responses. However, our
knowledge how Chlamydomonas achieves this is still at its
infancy. Here we show that ChR1 internalization occurs via
light-stimulated endocytosis. Prior or during endocytosis ChR1 is
modified and forms high molecular mass complexes. These are the solely
detectable ChR1 forms in extracellular vesicles (EVs) and their
abundance therein dynamically changes upon illumination. The
ChR1-containing EVs are secreted via the PM and/or the ciliary base. In
line with this, ciliogenesis mutants exhibit increased ChR1 degradation
rates. Further, we establish involvement of two cysteine proteases in
its turnover: CEP1, a papain-type C1A member, and a calpain. ΔCEP1
knock-out strains lack light-induced ChR1 degradation, whereas ChR2
degradation was unaffected. Low light stimulates CEP1 expression, which
is regulated via phototropin, a SPA1 E3 ubiquitin ligase and cAMP.
Further, mutant and inhibitor analyses revealed involvement of the small
GTPase ARL11 and SUMOylation in ChR1 targeting to the eyespot and cilia.
Our study thus defines the degradation pathway of this central
photoreceptor of Chlamydomonas and identifies novel elements
involved in its homeostasis and targeting.