Scientists uncover role of protein critical for activating DNA replication

Jan 07, 2010

Scientists at Cold Spring Harbor Laboratory (CSHL) have discovered how a protein long known to be an essential activator of DNA replication actually triggers this process in cells.

The , called DDK (for Ddf4-dependent ), is an enzyme that attaches molecules to other proteins to modify their activity. The CSHL team has found that DDK performs this operation, called phosphorylation, on a protein called Mcm4, specifically within a domain that acts as a built-in brake to prevent the from being unwound. The phosphorylation by DDK releases this brake, thus initiating the replication of unwound DNA strands.

"As DDK is often deregulated in human cancers, this new understanding of its role in DNA replication may help shape the development of new cancer therapies," explains CSHL President Bruce Stillman, Ph.D., who co-authored the study with colleague Yi-Jun Sheu, Ph.D. "Indeed recent studies have identified DDK inhibitors and they are now in clinical trials." The report was published in Nature on 7th January.

In multicellular organisms, the duplication of the DNA in starts at multiple sites, called origins, within the . For the genome to retain its integrity each time a cell divides, it's crucial that each origin "fires," or becomes active, just once and only during a timeframe in the cell cycle known as the S-phase.

A large number of proteins cooperate and interact with military precision to ensure this "once-only" condition. First, a group of proteins cluster at each origin site to form a pre-replication complex or pre-RC. The phosphorylation of some pre-RC components by DDK in turn recruits other proteins to these pre-RCs, converting them into pre-initiation complexes, or pre-ICs.

Over the last 15 years, Dr. Stillman's group has systematically uncovered many of the pre-RC and pre-IC proteins, and meticulously catalogued when and where each protein interacts with its collaborators. Having found out previously that DDK targets a multi-subunit protein complex called MCM, they've now narrowed down DDK's binding site to a domain within one of the subunits, Mcm4, where it phosphorylates a series of amino acids—protein building blocks—that otherwise inhibit Mcm4 from functioning.

The discovery of this self-inhibitory activity within Mcm4 and the finding that DDK is required to overcome it was a surprise, according to the authors. They propose that such complexity might have evolved in response to the importance of precision and accuracy of DNA replication.

"Although this is the only essential role for DDK under normal conditions, we have found that DDK takes on another task when the cell suffers DNA damage," says Dr. Stillman. In this hazardous situation, he and his colleague found, DDK activates an S-phase checkpoint mechanism that halts the DNA copying process and initiates DNA repair.

"This discovery of these distinct functions of DDK represents a key piece of the puzzle of how the initiation of is coordinated and controlled by kinase proteins," says Dr. Stillman.

Explore further: Environmental pollutants make worms susceptible to cold

More information: "The Cdc7-Dbf4 kinase promotes S phase progression by alleviating an inhibitory activity in Mcm4," appears in the 7th January issue of Nature. Doi:10.1038/nature08647

Related Stories

Common mechanisms for viral DNA replication

Jan 23, 2007

How DNA replicates is a critical question for understanding life. DNA replication remains difficult to investigate in eukaryotes,where it occurs within the confines of the double-membrane nucleus.

Chromatin remodeling complex connected to DNA damage control

Aug 09, 2007

When molecular disaster strikes, causing structural damage to DNA, players in two important pathways talk to each other to help contain the wreckage, scientists at The University of Texas M. D. Anderson Cancer Center report ...

Mutant mouse provides insights into breast cancer

Dec 11, 2006

By discovering a mutant mouse that is highly susceptible to mammary tumors, Cornell researchers have found a novel potential link between genetic defects in DNA replication (copying) and breast cancer.

Recommended for you

Environmental pollutants make worms susceptible to cold

6 hours ago

Some pollutants are more harmful in a cold climate than in a hot, because they affect the temperature sensitivity of certain organisms. Now researchers from Danish universities have demonstrated how this ...

A new quality control pathway in the cell

Sep 18, 2014

Proteins are important building blocks in our cells and each cell contains millions of different protein molecules. They are involved in everything from structural to regulatory aspects in the cell. Proteins are constructed ...

User comments : 0