Protein Localisation and Circadian Rhythms
Previous and Current research
Almost all organisms generate oscillations with a periodicity
of approximately 24 hours. These circadian clocks operate on
a cellular level and coordinate a wide variety of physiological
processes and behavioral activities. Circadian clocks are self-sustained
and persist robustly under constant conditions. Appropriate exogenous
stimuli such as light and temperature, so called zeitgebers,
entrain circadian clocks to synchronize the endogenous rhythms
with the environment. In the past years, substantial progress
has been made in the characterization of circadian systems on
the molecular level.
A central element of the circadian clock of the filamentous
fungi Neurospora crassa is frequency gen, FRQ.
Expression of FRQ depends on the transcription factors WC1 and
WC2, which form the heterodimeric complex (WCC). frq RNA
and FRQ protein levels oscillate under free running conditions
(constant darkness) with a period of 22 h. This oscillation is
supported by a negative feedback loop. frq mRNA is synthesized
in the late subjective night and reaches a maximum in the late
morning. Approximately 4 h later, FRQ protein levels reach a
maximum. FRQ protein enters the nucleus and, when expressed at
sufficiently high levels, represses its own synthesis. In the
course of the day, FRQ protein is progressively hyperphosphory-lated
and then degraded. When FRQ protein levels fall below a threshold,
frq RNA is synthesized again and a new circadian period
Our studies focus on the trafficking of FRQ and the WC proteins
between cytosol and nucleus, which regulates the interaction
of FRQ with WCC. Furthermore, we analyze the molecular processes
which determine the rate of FRQ degradation and thus the length
of the circadian day.
Projects for graduate students
Nuclear entry of FRQ and interaction of FRQ with WC2 and WC1
during a circadian day.
Identification of phosphorylation sites in FRQ, which determine
its subcellular localization.
Characterization of the casein cinase 1e of Neurospora and hyperphosphorylation
Merrow, M., Franchi, L., Dragovic, Z., Görl, M., Johnson,
J., Brunner, M., Macino, G. and Roenneberg, T. (2001) Circadian
regulation of the light input pathway in Neurospora crassa.
EMBO J., in press.
Merrow, M., Brunner, M. and Roenneberg, T. (1999) Novel assignment
of circadian function for the Neurospora clock gene frequency.
Nature 399, 584-586.
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