Dissecting the roles of the Tuberin protein in the subcellular localization of the G2/M Cyclin, Cyclin B1.

Tuberin is a major component of the protein regulatory complex known as the Tuberous Sclerosis Complex and plays a crucial role in cell cycle progression and protein synthesis. Mutations in the Tuberin gene, TSC2, lead to the formation of benign tumors in many organ systems and causes the Tuberous Sclerosis Complex disorder. Genotypes ranging from point mutations to large deletions in the TSC2 gene have been clinically characterized with a wide range of phenotypes from skin tumors to large brain tumors. Our lab has previously demonstrated that Tuberin can directly bind and regulate the timing of nuclear transport of the G2/M cyclin, Cyclin B1. Herein we study the consequence of one clinically relevant truncation in the Tuberin protein on cell cycle function. We demonstrate that exogenous expression of a fragment of the N-term region of Tuberin alters the subcellular localization of Cyclin B1 and increases cell proliferation. This adds to our body of information about the residues within Tuberin responsible for regulating the cytoplasmic retention of Cyclin B1 and supports the phenotypic data seen in the clinic with Tuberous Sclerosis Complex patients harbouring similar large deletions in Tuberin.

The Tuberin and Cyclin B1 complex functions as a novel G2/M sensor of serum conditions and Akt signaling.

A great deal of ground breaking work has determined that the Tuberin and Hamartin Complex function as a negative regulator of protein synthesis and cell cycle progression through G1/S. This is largely attributed to the GTPase activity of Tuberin that indirectly inhibits the mammalian target of rapamycin (mTOR). During times of ample nutrition Tuberin is inhibited by growth factor signaling, including direct phosphorylation by Akt/PKB, allowing for activation of mTOR and subsequent protein synthesis. It is well rationalized that maintaining homeostasis requires communication between cell growth (mTOR signaling) and cell division (cell cycle regulation), however how this occurs mechanistically has not been resolved. This work demonstrates that in the presence of high serum, and/or Akt signaling, direct binding between Tuberin and the G2/M cyclin, Cyclin B1, is stabilized and the rate of mitotic entry is decreased. Importantly, we show that this results in an increase in cell size. We propose that this represents a novel cell cycle checkpoint linking mitotic onset with the nutritional status of the cell to control cell growth.