Tim Hunt

Sir Tim Hunt, FRS, FMedSci is an English biochemist, born 19 February 1943 in Neston, Cheshire, England. He was awarded the 2001 Nobel Prize in Physiology or Medicine with Paul Nurse and Leland H. Hartwell for their discoveries of "Key regulators of the cell cycle".

The aim of the Cell Cycle Control Laboratory headed by Tim was to understand how cyclin-dependent protein kinases (CDKs) trigger cell cycle transitions, and how the timing of cyclin proteolysis is regulated.

Animal cells reorganise themselves as they prepare to divide. The nucleus disappears as its envelope disassembles. Chromosomes condense and shorten. The microtubule network reorganises to form the familiar bipolar spindle, and the Golgi apparatus fragments. The cells break contact with the substratum and round up. These dramatic changes are brought about by the phosphorylation of key components of these various subsystems by cyclin-dependent kinases.

Over the years, people in Tim's laboratory built up a list of substrates for these kinases, but lists are rarely informative or revealing of physiological significance. They allowed the lab to set a lower limit on the number of proteins that show mitotic phosphorylation. Another question, however, is: how completely phosphorylated are these substrates? Is it necessary for all of them to be completely phosphorylated to bring about the changes in form and function seen in dividing cells? Or is it enough if, say, half of all copies of a particular protein are modified? It is not clear if this is strictly necessary for the successful execution of mitosis. Such proteins completely revert to their interphase (de-phosphorylated) state when the cells return to interphase.

It turned out that at least one protein phosphatase, PP2A-B55, is regulated so that its activity is low in mitosis and high in interphase. The molecular basis of this regulation depends on a small, heat stable phosphorylatable phosphatase inhibitor that is highly specific for this particular form of phosphatase 2A.

Tim closed his lab at the Clare Hall Laboratories at the end of 2010, but remains an Emeritus Group Leader at the Francis Crick Institute.