Function of ABP195 member of a new small 'superfamily' of plant actin binding proteins; involvement in actin organisation and signalling

Plants are sedentary and have to respond to unique environmental and developmental signals in order to survive. The actin cytoskeleton is essential for life, is stimulus responsive and reorganises so that cells can divide and expand, generate organelle movement, polarise cell growth and thicken the cell wall; events that are required for healthy plant development i.e. for proliferation and growth, for maximising energy production, for the uptake of nutrients, for managing the transpiration stream, for reproduction and for protection against pathogen invasion. In order to perform these roles the actin network has to be able to respond to signals and reorganise, processes that are coordinated by actin associated proteins that anchor, crosslink or otherwise regulate this network. With its unique capabilities the actin cytoskeleton has become adapted to plant specific function by (1) tuning the function of its associated proteins, common to animals, fungae and plants, for its own purposes; there is a wealth of evidence of this from work in my lab and others and (2) the evolution of a set of plant specific associated proteins whose function on the actin network is unique to plants. This programme of research concerns a family of proteins that fit into category (2). The first indication that we had a new class of actin binding proteins in plants came from a screen for proteins localising to different compartments within a plant cell. This screen was performed by Oparka and colleagues (University of Edinburgh) and it revealed many new proteins locating to compartments such as Golgi, mitochondria, nucleus, plastids and the cytoskeleton. We have shown, as supporting data for this programme, that one of the proteins localising to the cytoskeleton localised to actin filaments. Identification of the actin binding domain in the amino -terminal region allowed us to identify 14 other proteins having the same domain but that the 15 proteins could be divided into 5 groups based on there carboxy-terminal sequence. This is not unlike the situation for known 'Superfamilies' of actin binding proteins where a common actin binding domain is attached to unique sequence and this unique sequence specifies its role with respect to actin in the cell. Preliminary data we present here plus utilisation of data from current databases suggest that one member of this superfamily is involved with the organisation of the actin network at the plasma membrane and one member is essential for normal plant development. The goal of this research is to understand how these two proteins contribute to the normal function of the actin cytoskeleton in plant development.

The identification of a new 'superfamily' of actin binding proteins highlights the fact that the structure and regulation of the plant actin network is unique compared to its animal and fungal counterparts. There are 15 members of this plant actin binding protein (ABP) superfamily, falling into 5 phylogenetic clades. All have a common N-terminal actin binding domain and each group has a specific C-terminus. The focus of this grant is to study a member of Group 1, and a member of Group 2. We have cell biological data on the Group 1 ABP that strongly suggests that this protein plays its role on the actin cytoskeleton at the plasma membrane, perhaps by crosslinking or helping anchor the actin network so that cell polarity is established/maintained. We have mutants that indicate that the Group 2 ABP is involved in embryo development, and that another member of this Group interacts and is phosphorylated by a receptor-like kinase. Taken together our current data indicate an important role for these proteins in actin organisation and signalling in plant development and the goal of this programme is to explain these roles. We will use a combination of cell biological, biochemical and genetic techniques to establish how the Group1 and Group 2 proteins contribute to the normal function of the plant actin cytoskeleton and how this is uniquely important for plant development.