Study Finds Protein Is Required For Human Chromosome Production

The new study appears in the most recent issue (Oct. 7) of the journal Cell.

Thestudy found that a protein called CPSF73 acts like scissors to cutstrands of histone messenger RNA (mRNA) in the cell nucleus. Thiscutting action produces the mRNA needed to create histone proteins thatcombine with DNA to form chromosomes.

Like all other proteins,histones are made when a specialized RNA molecule is "read" byribosomes, the cell's protein factories. The type of RNA, which relaysinformation from the DNA (inside the nucleus) to the ribosome (outsidethe nucleus), is called messenger RNA.

RNA that is not cut byCPSF73 is destroyed in the nucleus and never becomes messenger RNA,said Dr. William Marzluff, senior author of the study and Kenandistinguished professor of biochemistry and biophysics in UNC's Schoolof Medicine.

"This cutting of histone messenger RNA takes placeas growing cells prepare to divide and is absolutely necessary fortheir eventual division," Marzluff said. Histone proteins help organizeand compact within the nucleus the 6 billion nucleotides, or DNA bases,that make up the human genome – combinations of "A's," "T's," "G's" and"C's." Without histones, cells cannot survive.

Dr. ZbigniewDominski, associate professor of biochemistry and biophysics, has beensearching for the protein that cuts histone messenger RNA since joiningforces with Marzluff 10 years ago. He is the corresponding and leadauthor of the study.

When RNA is first made from DNA, it ispremature and cannot direct the synthesis of its corresponding proteinuntil it is processed into mature messenger RNA, which includes beingcut at a specific site, Dominski said.

"This is a very complexprocess that requires many proteins to bind to the RNA molecule andshow the cutting enzyme where to cleave the RNA," he added.

Dominskiwas able to duplicate, in a test tube, the histone mRNA processing thattakes place normally inside a cell's nucleus. However, the RNA cuttingreaction was so quick that he was unable to determine which of thecountless proteins inside the test tube was responsible.

"We seta trap by subtly changing the chemical makeup of the histone messengerRNA right where it is cut. This allows the protein to still come to theRNA but forces it to cut more slowly," Dominski said.

By slowingdown the reaction, Dominski had enough time to irreversibly attach theRNA cutting protein, or nuclease, to the RNA itself using ultravioletlight as a cross-linking agent. Once attached to the RNA, thelong-sought-after nuclease was trapped, allowing its subsequentidentification.

The discovery that the RNA cutting protein isCPSF73 was unexpected. This protein was already connected withprocessing of a completely different class of messenger RNA,polyadenylated mRNAs. These messenger RNAs serve as blueprints for allproteins other than the histones. "In terms of evolution, all messengerRNAs appear to be made with the aid of the same protein, CPSF73,"Marzluff said. "This suggests that the two mRNA processing mechanisms,for polyadenylated and histone mRNAs, are distantly related."

Theauthors further demonstrate that CPSF73 not only cuts the histonemessenger RNA molecule in two, but also then chews the unneeded portionof the histone mRNA molecule into pieces. It is rare to find two suchactivities within a single protein, Dominski said.

"From thepoint of view of understanding biology, our findings provide a unifiedmechanism for the synthesis of all messenger RNAs," Marzluff said.

UNCSchool of Medicine technician and co-author Xiao-cui Yang assistedDominski and Marzluff in this study. Their work was supported by agrant from the National Institutes of Health.