Two modes of exocytosis at hippocampal synapses revealed by rate of FM1-43 efflux from individual vesicles

Proteins that in cells specifically bind to growing microtubule plus ends (+TIPs) are thought to play important roles in polarization of the cytoskeleton. However, most +TIPs do not show a bias of their microtubule-binding behavior toward different subcellular regions. Here, we examine the dynamics of the +TIP CLASP in migrating PtK1 epithelial cells. We find that, although CLASPs track microtubule plus ends in the cell body, they dynamically decorate the entire microtubule lattice in the leading edge lamella and lamellipodium. Microtubule lattice binding is mediated by the COOH-terminal region of the CLASP microtubule-binding domain and is regulated downstream of Rac1. Phosphorylation of sites in the NH2-terminal part of the microtubule-binding domain by glycogen synthase kinase 3beta likely regulates the affinity of CLASPs for microtubule lattices. These results demonstrate the striking difference of the microtubule cytoskeleton in the lamella as compared with the cell body and provide the first direct observation of subcellular regulation of a microtubule-associated protein in migrating cells.