Now, a novel research reveals how a protein involved in cancer twists and morphs into different structures. The research focuses on a protein named nucleophosmin (NPM1). The protein has many functions and may interfere with cells' normal tumor suppressing ability when mutated. NPM1 has been implicated in cancers such as non-Hodgkin lymphoma and acute myelogenous leukemia.
Prior study had demonstrated that a section of NPM1, named the N-terminal domain (Npm-N), doesn't have a defined and folded structure. NPM1 morphs between two forms: a one-subunit disordered monomer and a five-subunit folded pentamer.
However, the mechanism behind this transformation were not clear. The researchers assumed this monomer-pentamer equilibrium could be essential for the protein's location and functioning in the cell.
To address this question, the researchers used an innovative combination of three techniques—single-molecule biophysics, fluorescence resonance energy transfer (FRET) and circular dichroism—which enabled them to study individual molecules and collections of molecules. Single-molecule approaches are very useful for such studies because they can identify essential information that remains hidden in conventional studies.
Importantly, the researchers discovered that the transformation can proceed through more than one pathway. In one pathway, the transformation begins when the cell sends signals to attach phosphoryl groups to NPM1. This modification, named phosphorylation, prompts the ordered pentamer to become disordered and likely causes NPM1 to shuttle outside the cell's nucleus. A meeting with a binding partner can mediate the reverse transformation to a pentamer.
When NPM1 does become a pentamer again under these conditions, which likely causes it to move back to the nucleolus, it takes a different path instead of just retracing its earlier steps.
The researchers compared these complicated transitions to the morphing of a "Transformers" toy, where a robot can become a car and then a jet. "Phosphorylation and partner-binding are like different cellular switches driving these changes," said Banerjee.
The new research also shows several intermediate states between monomer and pentamer structures—and that these states can be manipulated or "tuned" by changing conditions such as salt levels, phosphorylation and partner binding, which may explain how cells regulate the protein's multiple functions. Future studies could reveal the biological functions of these different structures and how they might be used in future cancer therapies.
The researchers noted that combining the three techniques used in this study, plus a novel protein-labeling technique for single-molecule fluorescence, could be a useful strategy for studying other unstructured, intrinsically disordered proteins (IDPs). IDPS are involved in a host of cellular functions, as well as neurodegenerative disease, heart disease, infectious disease, type 2 diabetes and other conditions.
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