Microtubule Plus End-Tracking Proteins and Their Activities in Plants

Microtubules form a dynamic system of filaments that have essential roles in all eukaryotic cells. Although microtubules are well-conserved across eukaryotic phyla, the organization and function of the microtubule cytoskeleton has evolved in ways that set higher plants apart from other organisms. The mechanisms that underlie these differences are topics of long-standing interest to plant cell biologists. A feature that is a key to microtubule function is their dynamic nature; they usually exist in states of growth or shrinkage. Their ability to grow, depolymerize, and then re-grow in new directions allows them to explore the cytoplasm and to rearrange into different configurations as needed. Microtubules also function in concert with a fleet of proteins that regulate microtubule activities within the cell. This chapter focuses on a specialized group of microtubule associated proteins (MAPs) that localize to the more active or plus ends of microtubules. From their position at the active end of the microtubule, these plus end-tracking proteins, or +TIPs, have a large influence on microtubule behaviour. They regulate growth and shrinkage rates and mediate interactions of the microtubule end with other proteins or structures in the cell. A number of +TIPs have been identified from several organisms including plants. The proteins form a structurally and functionally diverse group that have very little in common aside from an affinity for microtubule plus ends. Current models propose that these proteins form a dynamic +TIP network that is remodelled to include different sets of proteins depending on the needs of the cell. Here we discuss the repertoire of +TIP families found in plants. Plant cells have homologs corresponding to several proteins with plus-end binding activity in other organisms. Analyses indicate that there is some functional conservation amongst these proteins, although there are also cases where the plant proteins have been modified to function in different ways. In addition to conserved proteins, plants use at least one +TIP family that is not found in other eukaryotic genomes and there are some families that appear to be absent in higher plant lineages. The research to date suggests that plants have a somewhat modified +TIP network which is uniquely suited to the needs of the plant cell.