RB stands for retinoblastoma, one of the first diseases tied to a causal oncogene, now also called RB. For lack of time, this will be a very short post about a very lengthy story- how complicated one protein can be. The RB protein doesn't really do much on its own. It isn't an enzyme, or bind DNA, or do other dramatic things. But it binds to a lot of other proteins- 322 have been documented to date. And one protein that it binds to and represses, the transcription factor (family) E2F, is a key activator of cell division, promoting transcription of many other genes including cyclins and cyclin-dependent kinases that run the cell cycle. So RB is typically a key actor that keeps cells quiescent in G1 phase, the normal non-dividing state most of our cells are in. And this is how mutations in RB promote cancer, by removing this brake.
A recent paper expanded this story by investigating some of the regulation of the RB protein, which has at least 15 sites where it gets a phosphate group added (phosphorylated) by regulatory proteins called kinases. The most prominent regulatory kinases are the cell cycle dependent kinases, or CDK. Naturally when a cell does really want to divide, these function to turn RB off, via certain of these phosphorylations. The authors erased each of these phosphorylation sites, and then restored one at a time, asking what binds to them and their effect is. The upshot is that each site turned out to show a distinct pattern of downstream effects, indicating that different proteins bind more or less well to each phosphorylated form. These proteins include transcriptional regulators of a wide variety of kinds, and affect differentially the expression of key genes like BRCA1, 2, and MSH2, and processes ranging from DNA repair to oxidative phosphorylation to protein secretion.
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| Diagram of the sites of phosphorylation of RB by other proteins. The amino acid sequence goes from left to right, and functional regions of RB that bind to other proteins are colored. |
"Collectively, this mass spectrometric analysis identified 438 proteins with a statistically significant enrichment in complexes with at least one of the 16 forms of RB examined. The 22 proteins significantly enriched with all forms of RB included multiple E2F and DP [E2F partner] proteins."
Evolution has had several billion years to tinker with these systems. So while the solution sometimes has been elegance incarnate, (like the DNA molecule), other times it is a messy network of sprawling and mystifying scope. It is one reason why biologists will remain tied to their benches for decades to come.
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