Plant Molecular Biology 41: 151–158, 1999.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
Structural organization and interactions of COP1, a light-regulated
Magnus Holm and Xing Wang Deng
Department of Molecular, Cellular, and Developmental Biology, Osborn Memorial Laboratories, OML 301, Yale
University, P.O. Box 208104, 165Prospect Street, New Haven, CT 06520-8104,USA (
Received and accepted 13 August 1998
Key words: light regulation, nuclear translocation, seedling development, signal transduction, transcription
Arabidopsis seedling development follows contrasting patterns depending on ambient light conditions, photo-
morphogenesis in the light and skotomorphogenesis or etiolation in darkness. COP1 is a limiting or regulatory
component in mediating repression of photomorphogenesis in the absence of light. COP1 acts within the nucleus
in the dark, directly interacts and regulates speciﬁc transcription factors that are required for promoting photo-
morphogenesis.Light abrogates COP1 action and results in progressive nuclear depletion of COP1 with increasing
lightstimuli. COP1 contains multiple structural modules, which are responsible for interacting with distinct cellular
factors and play speciﬁc functional roles. We review the most recent progress in understanding the COP1 action
and propose speciﬁc models based on the recent studies.
Arabidopsis seedlings follow two distinct develop-
mental pathways depending on the presence or ab-
sence of light (Kendrick and Kronenberg, 1994; von
Arnim and Deng, 1996). Photomorphogenic devel-
opment occurs in the light and is characterized by a
short hypocotyl, differentiated and expanded cotyle-
dons and an active shoot apical meristem (Figure 1).
In darkness the seedling instead focuses its efforts
at reaching light. This is achieved, in a process
called skotomorphogenesis or etiolation, by elon-
gating the hypocotyl and maintaining closed, un-
expanded cotyledons, and an apical hook (Figure 1).
Genetic screens have identiﬁed genes acting as either
positive or negative factors in the light signaling net-
work regulating seedling development (McNellis and
Deng, 1995; Chory, 1997). Light is perceived by a set
of photoreceptors (i.e., phytochromes, cryptochromes
or UV-B receptors) which transduce signals inducing
photomorphogenesis. Seedlings with mutations that
disrupt the function of these photoreceptors or in pos-
itively acting downstream factors (such as the bZIP
transcription factor HY5) have an elongatedhypocotyl
in the presence of light. By contrast, mutations in
any one of 11 pleiotropic loci, the constitutively
FUS) genes, resulted in a reverse phenotype (Wei
and Deng, 1996, 1999). In darkness, the mutant
seedlings have short hypocotyls, differentiated and
expanded cotyledons, and de-repressed expression of
light-induced genes (Figure 1). Severe alleles in all
loci are lethal in adult stage, indicating that the genes
have an essential role in plant development.
Modular organization of COP1, a repressor of
The recessive nature of the cop/det/fus mutations sug-
gest that the group of genes deﬁned by the mutations
function as repressors of a default photomorphogenic
pathway (Chory et al., 1989; Deng et al., 1991).
Epistasis analyses have shown that this group of genes
act downstream of the multiple photoreceptors (Ang
and Deng, 1994; Chory, 1997). Furthermore, over-