Cell signaling is a highly complex series of events whereby cells perceive signals via surface receptor proteins and transduce those signals into a variety of cellular responses. One obvious feature of many of these signal transduction pathways is that they often contain a series of protein kinases - enzymes that transfer a phosphate group from ATP to a protein. Usually, the activity of the target protein is turned on or off by phosphorylation. When the signal is removed, kinases are inactivated and phosphatases remove the phosphate groups returning the target proteins to their original state. Closer examination of signal transduction pathways reveals that kinases often phosphorylate other kinases which phosphorylate other kinases and so on!!! This observation may lead one to ask how many protein kinases there are in a eukaryotic genome. Until recently, it was difficult to answer this type of question. Now it is easy, if you're into yeast.

The entire yeast genome is now completely sequenced. To make this vast amount of information available, various Internet sites were created around the world. A recent article in "Science" introduces readers to these sites. One site maintained by Proteome Inc. of Beverly MA is called the Yeast Protein Database. Among other things, it allows users to search the yeast genome using key words to obtain information about the proteins it encodes.

Now back to our question. A search of this database using the words "protein kinase" reveals 251 different genes! The entire yeast genome contains 5,885 potential genes so over 4% of those genes encode protein kinases! To be truthful, not all of these enzymes are involved in signal transduction pathways, but many are.

Questions: Why are there so many protein kinases involved in signal transduction? Alternatively, if each gene in the genome needed to be turned on by a separate signal transduction pathway, each involving several protein kinases perhaps a better question is why are there so few?!! Read about branching and convergence of signal transduction pathways to find a potential answer to these questions.

8/22/04 Copyright (C) 2004, Jonathan Monroe, monroejd@jmu.edu. All rights reserved.
URL: http://csm.jmu.edu/biology/courses/bio220/qotw9.html