Discovery Of A Motor Protein That Rewinds DNA

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Discovery Of A Motor Protein That Rewinds DNA

http://www.medicalnewstoday.com/articles/127659.php

Two biologists at the University of California, San Diego have

discovered the first of a new class of cellular motor proteins

that " rewind " sections of the double-stranded DNA molecule that

become unwound, like the tangled ribbons from a cassette tape,

in " bubbles " that prevent critical genes from being expressed.

" When your DNA gets stuck in the unwound position, your cells are in

big trouble, and in humans, that ultimately leads to death " said Jim

Kadonaga, a professor of biology at UCSD who headed the study. " What

we discovered is the enzyme that fixes this problem. "

The discovery represents the first time scientists have identified a

motor protein specifically designed to prevent the accumulation of

bubbles of unwound DNA, which occurs when DNA strands become

improperly unwound in certain locations along the molecule.

The UCSD researchers' findings, detailed in the October 31 issue of

Science, are also important because they provide biomedical

scientists with a greater understanding of the molecular mechanisms

that lead to a rare genetic disorder called Schimke immuno-osseous

dysplasia. The discovery will eventually allow medical researchers to

design future treatments for this devastating genetic disorder, which

causes strokes, congestive heart failure, kidney failure and death in

young children.

" We knew this particular protein caused this disease before we

started the study, " said Kadonaga. " That's why we investigated it. We

just didn't know what it did. "

What this protein, called HARP for HepA-related protein, did

astounded Kadonaga and Timur Yusufzai, a postdoctoral fellow working

in his laboratory. The two molecular biologists initially discovered

that this motor protein burns energy in the same way as enzymes

called helicases and, like helicases, attached to the dividing

sections of DNA. But while helicases use their energy to separate two

annealed nucleic acid strands - such as two strands of DNA, two

strands of RNA or the strands of a RNA-DNA hybrid - the scientists

found to their surprise that this protein did the opposite; that is,

it rewinds sections of defective DNA and thus seals the two strands

together again.

As a consequence, the UCSD biologists termed their new enzyme

activity an " annealing helicase. "

" We didn't even consider the idea of annealing helicases before this

study started, " said Kadonaga. " It didn't occur to us that such

enzymes even existed. In fact, we never knew until now what happened

to DNA when it got stuck in the unwound position. "

Now scientists who study the action of helicases on DNA and RNA have

an entirely new class of proteins to investigate.

" This will open up a whole new area of study, " said Kadonaga. " There

are very few enzymes known that alter DNA structure. And we've

discovered an entirely new one. This was not expected to happen in

the year 2008. We should have found them all by now. "

" I believe it's going to go beyond DNA. Just as there are DNA-DNA

helicases, there are RNA-DNA helicases and RNA-RNA helicases. So it

doesn't take a lot of imagination to foresee that there are probably

going to be RNA-DNA annealing helicases and RNA-RNA annealing

helicases. The field potentially can be fairly large. And as more and

more people discover additional annealing helicases, this field will

expand. "

Kadonaga and Yusufzai are already searching for more annealing

helicases, but they also plan to continue their studies of HARP.

" First, we want to do is find more of these proteins, so we're

looking for more right now, " said Kadonaga. " We also want to see what

other specific processes are affected by this particular protein,

HARP, in the cell. "