Organisms respond to environmental signals to balance the use of their limited resources. Receptors on the cell surface detect many diverse signals so that the plant cell can monitor its biotic and abiotic environment. Kinases are one class of receptor proteins that send signals into the cell by modifying themselves and other proteins. There are hundreds of kinases in plants, and they are vitally important for agriculture and plant breeders because they recognise hormones, microbes, carbohydrates and other signals that are associated with plant development and defence. Many kinases are predicted to bind carbohydrate (sugar) signals, but few of the signalling pathways or functions of these kinases have been described.
The structure of the carbohydrate-binding domain of malectin from Xenopus laevis defined a new protein domain that is structurally similar to but distinct from other carbohydrate-binding proteins such as the lectins (Schallus et al., 2008). Tertiary structures similar to malectin have been predicted for a number of plant proteins. Malectin domains are found in all kingdoms, while a tandem configuration of the malectin domain, the malectin-like domain, is specific to plants and algae (See figure). Within plants, extracellular malectin domains or malectin-like domains are most often paired with intracellular protein kinase domains, indicating that the malectin domains/malectin-like domains determine specificity of signal perception for receptor-like kinases in disease response and/or developmental pathways. The family size of both malectin domain-receptor-like kinases and malectin-like domain-receptor-like kinases is greatly expanded in higher plants. Given the recent annotation of this domain and its widespread occurrence in combination with a kinase domain, it is timely to consider how the malectin domain may contribute to the function of plant receptor-like kinases.
We are interested in the roles of the malectin domain receptor-like kinases that probably bind carbohydrates or sugar motifs that are connected to proteins (glycoproteins). These proteins are great candidates to help balance development/growth and defence in plants. While receptor-like kinases detect signals outside of the cell, there are two related motor proteins inside the cell that probably bind similar carbohydrate or glycoprotein molecules and actively move these molecules around the cell.
Although the malectin domain is found mostly in conjunction with a kinase domain (where it would be extracellular), this domain can also be found linked to a kinesin domain. In this case, the malectin domain probably binds to a carbohydrate or glycoprotein cargo that is moved along microtubules by the kinesin motor domain.
If we can identify how these two types of carbohydrate-binding proteins act in plant development, then this knowledge can potentially be used to increase crop yields. Before we can start to help crop breeders, we need to expand our understanding of how proteins detect and interact with different carbohydrates and the roles of carbohydrates in plant growth and defence.
Projects in the lab currently focus on:
- Functions of malectin domain receptor-like kinases in development
- Roles of malectin domain kinesins in cell division and embryogenesis