Opposing kinesin complexes queue at plus tips to ensure microtubule catastrophe at cell ends

Cell shape and polarity depend on proper microtubule (MT) regulation. In fission yeast, interphase MT arrays must scale with cell length to maintain nuclear positioning and division site accuracy. Prior research has shown that MT length is not fixed but adapts to cell size. However, the mechanisms by which MTs sense and respond to spatial cues remain unclear. Existing models propose that catastrophe factors accumulate based on MT length alone. Yet, no prior work had resolved how spatial positioning influences MT dynamics. This gap motivated investigations into how kinesin complexes might regulate MT plus ends in space and time. The role of Tea2/Tip1/Mal3 and Klp5/Klp6/Mcp1 in this process had not been fully established. Understanding these interactions could clarify how MT arrays are tuned to cellular geometry.

The study aimed to determine how microtubule length is regulated in fission yeast. Specifically, the researchers sought to identify the mechanisms by which MT plus ends respond to spatial cues at cell ends. They hypothesized that distinct kinesin complexes might compete to control MT dynamics. The goal was to test whether Tea2/Tip1/Mal3 and Klp5/Klp6/Mcp1 regulate MT plus ends in a spatially controlled manner. The study also aimed to clarify whether MT catastrophe is driven by length alone or by spatial positioning. By examining the interactions between these kinesin complexes, the researchers hoped to reveal how MT arrays adapt to cell size. This work could help bridge the gap between MT length and cell geometry in polarized cells.

The researchers used fission yeast as a model system to study microtubule regulation. They employed fluorescence microscopy to track microtubule plus ends in live cells. To label MTs, they used fluorescently tagged Mal3, a plus-end-tracking protein. Tea2 and Klp5/Klp6 were also tagged for visualization. Time-lapse imaging captured MT growth and catastrophe events. The team analyzed how Tea2/Tip1/Mal3 and Klp5/Klp6/Mcp1 interacted at MT plus ends. They observed whether Klp5/Klp6 could access MT ends when Tea2/Tip1/Mal3 was bound. Finally, they tested whether displacement of Tea2/Tip1/Mal3 at cell ends triggered MT catastrophe.

The Tea2/Tip1/Mal3 complex remained bound to growing MT plus ends, blocking access to Klp5/Klp6/Mcp1. Klp5/Klp6/Mcp1 accumulated behind Tea2/Tip1/Mal3 but could not reach MT plus ends while the stabilizing complex was bound. At cell ends, Klp5/Klp6/Mcp1 displaced Tea2/Tip1/Mal3 from MT plus ends. This displacement triggered MT catastrophe, ending MT growth. The study found that Tea2/Tip1/Mal3 restricted Klp5/Klp6/Mcp1 access during MT growth. However, at cell ends, Klp5/Klp6/Mcp1 overcame this restriction. The data suggest that MT catastrophe is not solely length-dependent but also spatially regulated. These findings indicate that kinesin complexes compete at MT plus ends to control MT dynamics.

The authors propose that microtubule length regulation in fission yeast is an emergent property of spatially regulated kinesin competition. They suggest that Tea2/Tip1/Mal3 stabilizes MTs during growth by blocking Klp5/Klp6/Mcp1 access. At cell ends, Klp5/Klp6/Mcp1 displaces Tea2/Tip1/Mal3, leading to MT catastrophe. The data support a model where MT length is not controlled by a single factor but by the balance between stabilizing and destabilizing kinesin complexes. The study shows that MT plus ends are dynamic sites of competition between kinesins. The findings challenge the idea that MT catastrophe depends solely on length. Instead, spatial positioning at cell ends plays a key role. The authors conclude that MT dynamics are shaped by both length and spatial regulation.