Researchers from UC Santa Barbara report the discovery of a novel function for WDR5, a protein known for its critical role in gene expression. In cells, WDR5 is a subunit of a five-protein complex. Mutations in members of this complex can result in childhood leukemia and other disorders affecting numerous organ systems in the body. The UCSB team worked with WDR5 in cultured human cell lines. “We found that when two cells divide, WDR5 is localized to a very interesting cellular structure called the midbody,” said Jeff Bailey, a graduate student in UCSB's department of molecular, cellular, and developmental biology (MCDB). “In the past, although associated with cell division, the midbody was considered 'junk,' but that has changed in the last decade. Now the midbody is believed to be important during stem cell differentiation.”
Baily was the lead author of the study (“WD Repeat-containing Protein 5 (WDR5) Localizes to the Midbody and Regulates Abscission”) published in the Journal of Biological Chemistry.
When a stem cell divides to produce a differentiated type of cell like a skin cell or a neuron, stem cells retain the midbody while differentiated cells do not. “This suggests that the midbody has important functions,” Bailey explained. “Also, when the midbody isn't cut correctly, the cells can re-fuse, creating one cell with two nuclei. This is thought to be part of what happens when a tumor forms.”
Conducted in the laboratory of MCDB associate professor Zach Ma, Ph.D., this new work involved the fusion of WDR5 to a green fluorescent protein molecule called EGFP. Although dense material within the midbody thwarts conventional methods of protein detection, the fluorescence of EGFP tethered to WDR5 revealed its location during cell division, or cytokinesis.
The researchers were surprised to find WDR5 in a part of the midbody called the dark zone. “It was very unexpected,” Bailey said. “The presence of WDR5 outside the cell nucleus gave us a clue about its function, which we tested,” added Dr. Ma.
The scientists found that not only did the protein localize in the midbody, it also contributed to abscission, the separation of two daughter cells at the completion of cytokinesis. In addition, WDR5 promotes the disassembly of midbody microtubules, the major structural components of the midbody that must be cleared before abscission can occur.
When the investigators artificially reduced the amount of WDR5 in cells, cytokinesis was substantially delayed and more cells failed to divide properly. “When histology is performed on a tumor, pathologists look for cells that have two nuclei,” Bailey explained. “This can indicate that cells within the tumor are failing to properly finish cytokinesis.”
Because a single protein can perform several distinct functions according to its location within a cell, it can be challenging to study one function without disrupting the others. Guided by previous structural studies, however, the UCSB team identified surfaces of the WDR5 “doughnut” that may be specific to its role in cell division.
“We have shed some light on the role of WDR5 in cytokinesis,” noted Dr. Ma, “which may in turn help us better understand the diverse array of physiological as well as pathological events related to malfunction of these proteins in the process of cell division,” as, for example, in the case of cancer.