Soft agar growth assays showing how the transformation of human mammary epithelial cells into primitive cancer cells (black dots) is suppressed in mutant cells lacking the WW domain (right) compared with wild-type cells (left).
From Ref. 1 © 2011 W. Hong
The transcriptional co-activator TAZ plays many roles in carcinogenesis. It not only promotes the migration, invasion, proliferation and creation of cancer cells, but also suppresses apoptosis—a form of programmed cell death that is meant to remove damaged or malfunctioning cells. Previous studies have demonstrated a direct correlation between the expression level of TAZ and the invasiveness of breast cancer cells.
TAZ is known to possess a domain called WW that binds proteins with certain structural motifs, most commonly the PPXY motif, and the domain has been suggested in recent studies to be critical for TAZ’s ability to transform healthy cells into cancer cells. The type of protein that the WW domain interacts with, however, has remained unclear. Wanjin Hong at the A*STAR Institute of Molecular and Cell Biology and co-workers have now identified the protein Wbp2 as a WW-binding protein that enhances TAZ’s transformational activity.
The researchers used a protein-isolating technique known as co-immunoprecipitation to identify proteins that might interact with the WW domain of TAZ. Among the candidates in the large-scale study were PPXY motif-containing proteins including Amot, AmotL1, Wbp2, YBX1 and Nudt21.
Wbp2 is known to interact with the WW domains of another transcriptional co-activator called YAP, which is quite similar to TAZ. For this reason, the researchers decided to focus their study on identifying the role of Wbp2 in TAZ regulation. They found that TAZ with a mutant WW domain failed to interact with Wbp2 and displayed lower transformational activity (see image). Joining Wbp2 to the carbon terminus of the mutant WW domain by force restored and enhanced TAZ’s ability to transform mouse fibroblasts. Reducing the expression level of Wbp2 suppressed TAZ’s ability to transform human mammary epithelial cells, whereas increasing the expression level of Wbp2 promoted the ability. The findings therefore suggest that Wbp2 regulates TAZ’s transformational activity through interaction with the WW domain.
Wbp2 is known to have three PPXY motifs in the carbon terminus. The second and third motifs are conserved among various species including humans, zebrafish, frogs, worms and flies, whereas the first motif is only conserved in some species. After testing Wbp2 with various combinations of mutated motifs, the researchers found that the second PPXY motif of Wbp2 is the most important for interaction with TAZ. “We discovered that Wbp2 plays a similar role in the fly, which suggests that Wbp2 has an evolutionarily conserved role in regulating cell proliferation,” says Hong. The A*STAR-affiliated researchers contributing to this research are from the Institute of Molecular and Cell Biology.
