Photic entrainment of the Drosophila circadian clock
Copyright
Amita
Sehgal (Home
Page), Wei Song, Fang-Ju Lin and Nirinjini
Naidoo
Howard
Hughes Medical Institute, University of Pennsylvania
Medical School, Philadelphia, PA 19104
In the fruit
fly, Drosophila melanogaster,
circadian (~24 hour) rhythms of eclosion and
rest:activity are controlled by an
endogenous clock that is generated through
the coordinated action of specific genes.
Dominant among these clock genes are the
period (per) and timeless (tim) genes, both
of which encode RNAs and proteins that cycle
with a circadian rhythm. The two proteins
(PER and TIM) interact directly and feed
back to negatively regulate synthesis of
their own mRNAs. The feedback loop thus
generated is thought to constitute the
molecular basis of the clock. Entarinemnt of
this molecular clock to the day:night cycle
is achieved through light-induced
degradation of the TIM protein. We recently
showed that TIM is degraded by the
proteasome in response to light (1). Prior
to its degradation by the proteasome, TIM is
phosphorylated on tyrosine residues and
ubiquitinated. In order to determine the
mechanisms that transduce light to TIM, we
investigated the role of cryptochrome (CRY)
in TIM degradation. Cryptochrome is a flavin-binding
molecule that is thought to function as a
circadian photoreceptor in Drosophila. In
support of this, flies carrying a mutation
in the flavin-binding site of CRY show
deficits in entrainment (2). In addition,
TIM oscillations in the presence of
light:dark cycles are abolished in these
flies such that TIM levels remain high
during the day (2). This indicates that
light-induced TIM degradation is affected.
We used a cell culture system to investigate
the role of CRY in TIM degradation. We found
that CRY, but not CRY b increases TIM
ubiquitination in a dose-dependent manner.
Overexpression of CRY, however,
downregulates TIM ubiquitination perhaps
because CRY acts as a dominanat negative
under these conditions. After transducing
the photic signal which leads to TIM
ubiquitination, CRY itself is degraded by
the proteasome. These data and other
relevant recent studies will be discussed.
Index terms:
Drosophila melanogaster, circadian
rhythms, TIM, CRY, proteasome.
Copyright:
The copyrights of this original work
belong to the authors (see right-most
box in title table). This abstract
appeared in Session 18 – REPRODUCTION
AND DEVELOPMENT Symposium and Poster
Session, ABSTRACT BOOK II –
XXI-International Congress of
Entomology, Brazil, August 20-26, 2000
and in
Science
285, 1737-1741 (1999).
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