doi:10.1038/nindia.2008.297 Published online 13 October 2008
Researchers have discovered the antagonistic functions of two protein variants coded by one gene resulting in the fatal neurodegenerative disorder Lafora disease1.
One of the reasons of unexpectedly low number of protein coding genes (roughly about 35,000) in humans could be the ability of a single gene to code more than one distinct protein species through a process known as alternative splicing. During gene expression, different segments of the primary messenger RNA are separated and connected (spliced) to produce a functional RNA that undergoes a process known as translation to produce the desired protein.
However, for some genes, the messenger RNA undergoes more than one type splicing so that a variety of proteins are produced to perform distinct cellular functions. Nearly 70% of the genes in humans are believed to undergo alternative splicing. Misregulation during this process is implicated in a number of human diseases including cancers and neurological disorders. The biology of alternative splicing and the distinctive function of protein variants coded by the same gene are yet to be fully understood.
The researchers discovered that the strikingly antagonistic functions of protein variants coded by the EPM2A gene resulted in Lafora disease. The two protein variants coded by the alternatively spliced transcript of one single gene — EPM2A — occupy distinct compartments inside the cell. The major isoform (identified as laf331) remains in the cytoplasm whereas the minor variant (laf317) targets nucleus in addition to the cytoplasm.
"While laf331 is an active enzyme that removes phosphate groups from its substrate, laf317 is an inactive enzyme," says lead researcher Subramaniam Ganesh, an associate professor in the department of Biological Sciences and Bioengineering at Indian Institute of Technology, Kanpur. However, laf317 can compete with its active cousin for the substrate, thereby inhibiting its enzymatic activity by forming a complex. Thus the 'inactive' variant of the EPM2A gene appears to serve as dominant-negative regulator for the 'enzymatically active' variant.
The researchers further showed that the disease associated mutant forms of laf31 were unable to interact with laf317. This suggests that laf331-specific mutations might affect laf317 functions as well. These findings reveal a novel mechanism for the EPM2A gene function, regulated by alternative splicing, in normal as well as disease conditions.