News from the Physiology Course: “Pile-Up” of Proteins at Microtubule Ends Helps Scaffold the Cell

The 2017 MBL Physiology course participants. Marileen Dogterom is 4th from left in back row (in headband), Louis Reese is 1st from left in 3rd row (in orange sweater) and Matt King is 6th from right in 3rd row (with glasses on head).

By MBL Communications February 01, 2023

A recent study in Nature Cell Biology got a big push from participants in the MBL Physiology course -- and draws upon MBL discoveries that go back for decades.

The paper concerns microtubules, the stiff filaments inside cells “that are analogous to rigid 2 x 4s that hold up the cell,” says Matthew King, a co-author on the paper who was a student in the 2017 Physiology course from Princeton University.

“We had previously observed that the tips of microtubules seemed to be coated with a sort of protein ‘glue’ that would cause them to stick to the membrane of the cell,” says King. “Or to extend the analogy, to allow the 2 x 4s to adhere to the house’s roof.”

But what exactly was happening here?

In their paper, the team works out a physical description of how three proteins accumulate on the tip of a microtubule. These tip proteins, they discovered, are actually a collective material, “an association of proteins that aren’t present in fixed ratios, but come about from multivalent interactions,” says Marileen Dogterom of Delft University of Technology, the paper’s senior author and a faculty member in 2017 Physiology.

And it’s a motor protein that drives proteins to the tip, where they pile up. (Motor proteins were co-discovered by Ron Vale and others at the MBL in the mid-1980s).

“We cannot explain the observed network theoretically without this 'pile-up',” King says.

Droplet transfer between two microtubules. Under crowding conditions, the three tip proteins formed droplets at the microtubule tip. These droplets spread on the microtubule lattice and fuse at the plus-end. Scale bar, 5 µm. Credit: Maan et al, Nature Cell Biology, 2022.

"Like a Huge Dollop of Glue"

Dogterom had come to the MBL Physiology course with the observation that the three microtubule tip proteins (Mal3, Tea2 and the aptly named Tip1) could be collectively transferred from a microtubule to a cell-membrane mimic.

“We found this observation puzzling because it suggested that those proteins collectively constitute a material,” King says.

King was then a PhD candidate at Princeton studying phase separation processes in cells with Sabine Petry and Clifford Brangwynne, both of whom have long associations with MBL as faculty and investigators. Phase separation, in which molecules spontaneously form liquid-like droplets inside cells, was first observed in the 2008 Physiology course.

King teamed up in the course with Louis Reese, a post-doc from Dogterom’s lab, to investigate. They decided to increase crowding to pack the tip proteins closer together.

“Under these conditions, the tip structure got bigger; in fact, it resembled a liquid-like droplet – like when a kid defies their art teacher and puts a huge dollop on glue on the end of their popsicle stick!” King says. “This observation was the start of our investigations of the tip proteins as a collective material.”

The MBL awarded King a post-course fellowship so he could follow up on these intriguing results with Dogterom and Reese. The next spring, King spent six weeks in Dogterom's lab in Delft, where they continued the research that would lead to this publication.

Citation:

Renu Maan et al. (2022) Multivalent interactions facilitate motor-dependent protein accumulation at growing microtubule plus-ends. Nature Cell Biology, DOI: 10.1038/s41556-022-01037-0