During animal cell division, a highly synchronized and tightly regulated dance of chromosomes takes place, ensuring the chromosomes split correctly into the two cells. Spindle fibers — complex machinery responsible for choreographing this dance — form from each end of the cell and push and pull the chromosomes through the process. But scientists still do not fully understand how cells control where and when the spindle fibers themselves form.
Now, researchers from the Okinawa Institute of Science and Technology (OIST) and the University of California, San Diego, have revealed the exact mechanism underlying how a key protein, called SPD-5, helps regulate where and when spindle fibers assemble in the roundworm, C. elegans. Published in Science Advances , the study shows that SPD-5 is activated through a precise, step-by-step process that changes the protein's shape, unlocking the binding sites needed to begin forming spindle fibers. Their findings reveal key insights into the fundamentals of how cell division is regulated, and in the future, could open new avenues into treating diseases caused when cell division goes wrong.
"When you look at cell division under a microscope, you can see that the spindle fibers only grow at a specific time during cell division and from limited places — one of which is the centrosome," says first and co-corresponding author, Midori Ohta, who is currently a Buribushi Fellow and Principal Investigator of the Centrosome Dynamics and Evolution Group at OIST.
"Centrosomes are the main spindle construction site and my key area of interest," continues Ohta. "If they aren't formed or regulated properly, chromosomes can segregate incorrectly. This can lead to an abnormal number of chromosomes within a cell, cancer, or developmental diseases like microcephaly, which affects brain development."
Unravelling the steps of SPD-5 activation
Centrosomes consist of two core structures called centrioles, surrounded by the pericentriolar matrix — a cloud of proteins that greatly expands when a cell divides. In C. elegans, this cloud is mainly formed by SPD-5, which plays a vital role by binding to and activating γ tubulin complexes. Once in place, the γ tubulin complexes act as assembly sites, or starting points, for the microtubules that make up the spindle fibers.
"A key question was to figure out how SPD-5 is activated, so that it only binds to and activates γ‑tubulin complexes at the centrosome and not in other parts of the cell," says Ohta.
The researchers found that before SPD-5 is activated, its structure keeps it in a built-in "off" state, folding in on itself, like a clenched fist. The two parts of SPD-5 responsible for grabbing a γ tubulin complex block each other, preventing it from doing its job too early.
But this changes when the cell gets ready to divide. As SPD-5 is incorporated into the growing protein cloud at the centrosome, another protein adds small chemical phosphate tags to SPD-5. This reshapes SPD-5, loosening its folded structure like a hand opening up, and freeing up one of the binding sites to attach onto a γ tubulin complex. That interaction triggers a further shift in the protein's shape, releasing the second site so that it can also latch onto the γ tubulin complex, making the interaction stronger and more stable. This step-by-step activation bypasses SPD-5's built-in safety lock, ensuring microtubules form only at the right place and time.
From worms to humans
While this study was carried out in C. elegans, the basic structure of centrosomes is similar across animals, including humans. Building on these findings, Ohta and colleagues now plan to investigate the CDK5RAP2 family — the human counterparts of SPD‑5 — to find out whether they are controlled in the same step‑by‑step way.
"These proteins play an essential role in the brain, and mutations in them can lead to developmental disorders such as microcephaly," says Ohta. "By understanding the fundamental mechanisms that control spindle formation with such precision, we hope to gain important insight into how errors in this process can contribute to human disease."
