Trends in Plant Science
Calmodulin, calmodulin-related proteins and plant responses to the environment
Section snippets
Structural basis of Ca2+-dependent functions of calmodulin
Most proteins that function as intracellular transducers of Ca2+ signals contain a common structural motif, the `EF hand'[5], which is a helix–loop–helix structure that binds a single Ca2+ ion. These motifs typically occur in closely linked pairs, interacting through antiparallel β-sheets[5]. This arrangement is the basis for cooperativity in Ca2+ binding. The superfamily of EF-hand proteins is divided into several classes based on differences in number and organization of EF-hand pairs, amino
Unique calmodulin-related proteins: a possible basis for distinct functions
All plants seem to possess types of calmodulins that are very similar (close to 90% amino acid sequence identity) to mammalian calmodulin (Fig. 3). However, an intriguing finding in recent years is that plants possess different types of calmodulin-related proteins that are distinct from those in other organisms. The terms calmodulin-related proteins and calmodulin isoforms are not clearly defined, and thus here we have considered as calmodulin-related proteins only those proteins that have at
Dynamic expression of calmodulin-related genes
Many plant species have been shown to possess calmodulin multigene families composed of several genes encoding an identical protein as well as other genes encoding various calmodulin-related proteins. For example, in wheat, at least seven genes encode the conserved calmodulin TaCaM-I; at least two other genes encode TaCaM-II, which differs from TaCaM-I in just two conserved amino acid substitutions; and at least one other gene encodes TaCaM-III, a novel calmodulin-related protein lacking the
The diversity of plant calmodulin-binding proteins
In animals, over 25 calmodulin targets have been identified, including kinases, receptors, ion channels and G-proteins[21]. In plants, relatively few calmodulin-regulated proteins have been isolated, even though calmodulin has been implicated in a range of cellular processes as diverse as responses to pathogens[22], gravitropism23, 24, light[3]and cold[17]. However, much of the evidence for the involvement of calmodulin in plants is based on the use of pharmacological agents that function as
Future perspectives
The existence of multigene families of calmodulin and calmodulin-related proteins in plants and the growing list of known targets of calmodulin suggest a complex Ca2+-mediated regulatory network controlling development and responses to the environment. Future studies should aim to dissect the specific roles of members of the calmodulin superfamily, their subcellular distribution and the physiological relevance of their interaction with target proteins. Different approaches include affinity
Acknowledgements
We thank V. Sobolev for assistance in preparing three-dimensional structure computer images. We also thank M. Apse, T. Arazi, A. Danon, Y. Eyal and P. McCourt for critical reading of the manuscript. W.A.S. is the recipient of a Canadian National Science and Engineering Research Council postdoctoral fellowship.
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