CLIP-associating protein (CLASP) 1 and CLASP2 are mammalian microtubule (MT) plus-end binding proteins, which associate with CLIP-170 and CLIP-115. Using RNA interference in HeLa cells, we show that the two CLASPs play redundant roles in regulating the density, length distribution and stability of interphase MTs. In HeLa cells, both CLASPs concentrate on the distal MT ends in a narrow region at the cell margin. CLASPs stabilize MTs by promoting pauses and restricting MT growth and shortening episodes to this peripheral cell region. We demonstrate that the middle part of CLASPs binds directly to EB1 and to MTs. Furthermore, we show that the association of CLASP2 with the cell cortex is MT independent and relies on its COOH-terminal domain. Both EB1- and cortex-binding domains of CLASP are required to promote MT stability. We propose that CLASPs can mediate interactions between MT plus ends and the cell cortex and act as local rescue factors, possibly through forming a complex with EB1 at MT tips.

The tight junction (TJ) strand is a linear proteinaceous polymer spanning plasma membranes, and each TJ strand associates laterally with another TJ strand in the apposing membranes of adjacent cells to form ''paired'' TJ strands. Claudins have been identified as the major constituents of TJ strands, and when exogenously expressed in L fibroblasts, they polymerize into paired strands, which are morphologically similar to paired TJ strands in epithelia. Here, we show that a fusion protein of GFP with claudin-1 can also form similar paired strands in L fibroblasts, allowing us to directly observe individual paired claudin strands in live cells in real time. These paired strands showed more dynamic behavior than expected; they were occasionally broken and annealed, and dynamically associated with each other in both an end-to-side and side-to-side manner. Through this behavior of individual paired claudin strands, the network of strands was reorganized dynamically. Furthermore, fluorescence recovery after photobleaching analyses revealed that claudin molecules were not mobile within paired strands. Although these observations are not necessarily representative of TJ strands per se in epithelial cells, they provide important information on the structural and kinetic properties of TJ strands in situ with significant implications for barrier function of TJs. T he tight junction (TJ) is one mode of cell-to-cell adhesion in epithelial and endothelial cells. TJs seal the cells to create a primary barrier to the diffusion of solutes across the cellular sheet, and also function as a boundary between the apical and basolateral membrane domains to produce their polarization (1-5). On ultrathin section electron microscopy, TJs appear as a series of discrete sites of apparent fusion, involving the outer leaflets of the plasma membranes of adjacent cells (6). On freeze-fracture electron microscopy, TJs appear as a set of continuous, anastomosing intramembranous particle strands (TJ strands; ref. 7). These observations led to our current understanding of the three-dimensional structure of TJs; each TJ strand associates laterally with another TJ strand in apposing membranes of adjacent cells to form ''paired'' TJ strands, where the intercellular space is completely obliterated (reviewed in ref. 5).To date, three distinct types of integral membrane proteins have been shown to be localized at TJs; occludin (8, 9), junctional adhesion molecule (10), and claudins (11). Occludin, an Ϸ65-kDa integral membrane protein with four transmembrane domains, was identified as the first component of TJ strands. However, several studies including gene knockout analyses revealed that TJ strands can be formed without occludin (12-15). JAM with a single transmembrane domain was recently shown to associate laterally with TJ strands, but not to constitute the strands per se (16). In contrast, claudin is now believed to be a major constituent of TJ strands (reviewed in refs. 5 and 17). Claudins with molecular masses of Ϸ23 kDa comprise a multigene family...