After nearly four decades of research, Mayo Clinic scientists have revealed the molecular structures of protein kinase C beta (PKCβ), a key protein linked to cancer and neurological diseases. The findings, published in Nature Communications, provide the first detailed view of how the protein works and how the breast cancer drug endoxifen can target this protein.
PKCs are a family of proteins that help cells communicate. They act as molecular switches that regulate cell growth, survival and behavior. Because they play a role in many diseases like Alzheimer's and cancers like breast, lymphoma and colorectal, scientists have long viewed them as promising drug targets. However, without understanding their structure, designing effective therapies has been difficult.
"For decades, scientists have been trying to understand how these proteins function," says study co-author Matthew Goetz, M.D., a medical oncologist at Mayo Clinic Comprehensive Cancer Center. "These findings create new opportunities to develop more precise therapies for cancer and other diseases."
Solving a 4-decade-long mystery
Since PKC was first discovered in the 1980s, scientists have been unable to determine the structure of full-length human PKC enzymes, limiting efforts to understand how they function and how they might be targeted therapeutically.
Led by senior author Matthew Schellenberg, Ph.D., Mayo Clinic researchers overcame this mystery by producing human PKC enzymes that more closely resemble their natural state than proteins generated using traditional insect cell approaches. Their method revealed the structures of human PKCβ1 and PKCβ2.
"By producing the protein in human cells, we were able to obtain high-quality material that enabled us to finally see how this enzyme is organized and regulated," says Dr. Schellenberg, a molecular biologist at Mayo Clinic. "Now, we can begin investigating how changes in these proteins contribute to disease and how new therapies might selectively influence their activity."
How a breast cancer drug inhibits PKCβ
It has been known for decades that PKCβ becomes activated when it interacts with lipid membranes inside cells, but it was not known how this could happen. Structural studies revealed that when membrane lipids bind to PKC, they act like a molecular lever, shifting the enzyme from a closed, inactive state to an open, membrane-bound active state. These membranes trigger changes in the protein, exposing its active site and switching it on.
The researchers then combined structural biology, biochemistry and cellular studies to understand how endoxifen affects PKCβ. They found that endoxifen inhibits PKCβ through an allosteric mechanism, meaning it changes the protein's behavior without directly competing for its active site. The drug appears to stabilize PKCβ at cellular membranes, triggering changes that ultimately lead to its degradation.
"This mechanism is fundamentally different from previous PKC inhibitors that have been tested over the years," Dr. Goetz says. "That distinction may help explain why endoxifen shows biological effects that earlier compounds did not."
Implications for precision medicine
The findings establish a framework for understanding how different PKC family members function in health and disease. Some PKC isoforms may promote tumor growth while others may suppress it. The PKC family includes 10 related proteins, each with distinct roles. Determining when each protein should be activated or inhibited has remained a major unanswered question.
"This study gives us the tools to ask those questions in a much more sophisticated way," Dr. Schellenberg explains. "We can now investigate how different PKC proteins contribute to cancer and design drugs that target the right protein in the right context."
Mayo Clinic researchers are currently studying endoxifen in premenopausal women with estrogen receptor-positive breast cancer and investigating whether its effects on PKCβ contribute to its anticancer activity. The team is planning future work to expand beyond PKCβ to all 10 members of the PKC family, seeking to understand how each enzyme functions and responds to therapeutic compounds in its own unique way.
"We've opened a new door," says Dr. Goetz. "For the first time, we can see how these proteins are organized, how they function and how they may be targeted with greater precision. That understanding could help guide the next generation of therapies."
For a complete list of authors, disclosures and funding, review the study.
