The abundance of certain subcellular compartments is dramatically altered by changes in metabolic state; therefore it seems reasonable that organelle biogenesis in general would be responsive to changes in cell physiology. The constitutive secretory pathway, essential for the synthesis and delivery of proteins and lipids to the cell surface as well as for the biogenesis of the organelles of the endomembrane system, is a highly dynamic network of membrane-bound compartments and thus is a prime target for physiological regulation. While extensive research has yielded a detailed molecular description of a number of individual membrane trafficking steps, research aimed at understanding how these pathways are regulated and integrated is just beginning. The larger goal of our resesarch is to elucidate the regulatory mechanisms that allow the secretory pathway to respond to changes in cell physiology. At one end of the spectrum, we hope to identify the intracellular signals, presumably components of signal transduction pathways, that mediate alterations in membrane trafficking. At the other end of the spectrum, we hope to determine how the signals, that are generated in response to physiological stimuli, impinge on transport. This will consist of identifying the transport factors whose activities are modulated by changing phsyiological conditions as well as investigating the mechanism by which their activities are modulated.

To identify regulators of membrane trafficking, we have focused on the reaction pathway underlying the formation of endoplasmic reticulum (ER)-derived transport vesicles - the first defined step in the early secretory pathway. The formation of transport vesicles from the ER is mediated by a group of cytoplasmic coat proteins termed coat complex II (COPII). Using an heterologous in vitro system consisting of purified constituents of the minimal COPII machinery and semi-intact cells, we demonstrated the requirement for a previously unidentified cytosolic activity in COPII assembly at mammlian ER exit sites. We recently purified this activity, using conventional chromatographic procedures, and identified it using mass spectrometry. The protein identified is the Nm23H2 isoform of Nucleoside Diphosphate Kinase. Nm23 is a family of enzymes whose best characterized function is to maintain nucleoside triphosphate pools, but whose members have recently been identified as potential regulators of diverse cellular activities, including clathrin-mediated endocytosis from the plasma membrane. Studies are currently underway to demonstrate the function of this kinase in the formation of COPII vesicles at the ER in intact mammalian cells. As a potential regulator of COPII assembly, the Nm23H2 kinase is prime candidate for linking the early secretory pathway to physiological stimuli.

Kapetanovich L, Baughman C, Lee TH. Nm23H2 Facilitates Coat Protein Complex II Assembly and Endoplasmic Reticulum Export in Mammalian Cells. Molecular Biology of the Cell 2005;16(2):835-48.

Lee TH, Linstedt AD. Potential role for protein kinases in regulation of bidirectional endoplasmic reticulum-to-Golgi transport revealed by protein kinase inhibitor H89. Molecular Biolology of the Cell 2000; 11(8):2577-90.

Lee TH, Linstedt AD. Osmotically induced cell volume changes alter anterograde and retrograde transport, Golgi structure, and COPI dissociation. Molecular Biology of the Cell 1999; 10(5):1445-62.

Lee TH, Turck C, Kirschner MW. Inhibition of cdc2 activation by INH/PP2A. Molecular Biology of the Cell 1994; 5(3):323-38.

Lee TH, Solomon MJ, Mumby MC, Kirschner MW. INH/PP2A is a negative regulator of MPF. Cell 1991; 64(2):415-23.