New & Noteworthy

Not any Cyclin Will Do

March 16, 2012

Clb5 is important for DNA synthesis before sporulation.

Getting through a cell cycle is a complicated process. All sorts of proteins need to work and stop working at the right times in the right places to get the DNA copied, get the cells growing larger, have the cells divide and so on.

Key regulators in this process are the cyclins and their dependent kinases. Different cyclins are expressed at different points in the cell cycle, at which time they direct their cyclin-dependent kinase (CDK) to the appropriate subset of proteins to be phosphorylated. A big part of cell cycle regulation, then, comes from when a cyclin is expressed. But this is not the whole story.

In a study out in this month’s issue of GENETICS, DeCesare and Stuart showed that at least for the B-type cyclin Clb5 in the yeast S. cerevisiae, timing isn’t everything. And even more unexpectedly, they found that a key part of this cyclin’s specificity comes from its N terminus.

In yeast, Clb5 is involved in premeiotic DNA synthesis. Many researchers had previously argued that any B-type cyclin expressed at the right time would be sufficient to promote this function. DeCesare and Stuart were able to show that this was not the case by putting two different cyclins, Clb1 and Clb3, under the control of the CLB5 promoter. These cyclins were now expressed at the right time but neither could substitute for Clb5.

The authors next set out to discover what part of Clb5 conferred this specificity by creating chimeric versions of Clb3 and Clb5. They identified two regions in Clb5 important for premeiotic DNA synthesis — a hydrophobic patch and the N terminus.

The hydrophobic patch was expected; this region is highly conserved in all cyclins and has previously been shown in to be involved in interacting with protein substrates. But the N terminus was a surprise. It was thought to be involved primarily in cyclin stability and/or subcellular localization and not protein-protein interactions.

The authors were not able to identify which specific part of the N terminus of Clb5 was involved in conferring specificity. In their experiments, there was a gradual decline in the ability of the Clb3-Clb5 chimera to promote premeiotic DNA synthesis as more and more of Clb5 was replaced with Clb3. It is as if the whole region is involved in determining specificity.

And the decreasing ability of the Clb3-Clb5 chimera to induce premeiotic DNA synthesis was not due to the loss of kinase activity. When paired with Cdc28 (also known as Cdk1), all of the chimeras in the experiment were equal or even more active than the wild type Clb5/Cdc28 pair.

What it looks like is happening is that Clb5 uses both its hydrophobic patch and its N terminus to bring appropriate proteins to Cdk1 for phosphorylation. Different parts of each region are used to interact with different subsets of proteins involved in premeiotic DNA synthesis. At least for Clb5 and premeiotic DNA synthesis, it looks like not any cyclin will do.

by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics