Multifunctional annexins

Annexins are soluble proteins that undergo conditional association or insertion into membranes. Plants contain several isoforms, each of which may be capable of supporting more than one in vitro activity such as actin binding, phosphodiesterase activity. peroxidase activity. and cation transport. Enzymatic activities are modulated by lipid binding, Ca2+ and S-glutathionylation. A given annexin can occupy diverse positions in cells, including the apoplast and organelles, with membrane association and expression often as a consequence of perception of a stimulus (for example, salinity, nodulation) that may involve reactive oxygen species. The ability to translocate Ca2+ in vitro identifies annexins as a novel class of plant ion transporters that could account for channel activities in plasma- and endo-membranes and suggests roles in plant signalling and development. Studies on loss of function or overexpressing lines firmly implicate annexins as participating in the regulation of drought and salinity stress responses. How annexins operate in vivo, in terms of localisation and protein function now needs to be determined. With several tiers of regulation (space, time, post-translational modification) potentially operating on the soluble and membrane populations, annexins are complex components of plant cell Ca2+ networks. (C) 2009 Published by Elsevier Ireland Ltd.