The Department of Biomedicine

BBB seminar: Jennifer Lippincott-Schwartz

The organization and function of the Golgi apparatus as a membrane transport system

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Jennifer Lippincott-Schwartz
Cell Biology and Metabolism Branch, NICHD, National Institutes of Health, Bethesda, MD, USA

The Golgi apparatus plays a central role in protein processing and sorting within the secretory pathway. In mammals, it is comprised of a series of flattened cisternae arranged as a stack. Within the stack, resident Golgi enzymes are distributed in a polarized fashion with early-acting enzymes localized in the cis-most cisternae and later-acting enzymes found in the trans-most cisternae. The enzymes retain this polarized distribution in the face of continuous movement of cargo through the Golgi system en route to the cell surface or other cellular destinations. Two general models have been proposed to account for these properties of the Golgi. The first model - cisternal maturation - proposes that the numerous cisternae comprising the Golgi act as cargo carriers in a process of cisternal progression, in which cargo-laden cisternae move or mature from the cis (entry side) to the trans (exit side) of the Golgi stack. Resident Golgi enzymes are retained with the Golgi by being shuttled in vesicles that move backwards across the stack of cisternae. In the second model - vesicle transport - the cisternae are stable elements with cargo transferred between them by small vesicles that move forward across the stack. Golgi enzymes are retained within a given cisternae and excluded from transport vesicles. As some types of cargo have been found to move through the Golgi stack faster than others, and Golgi vesicles contain both secretory cargo molecules and Golgi enzymes, these hypotheses have been inadequate in their customary forms to explain Golgi behavior and organization.

Here, we use live cell imaging approaches with GFP chimeras to develop a detailed quantitative model of intra-Golgi traffic that accounts for the unique characteristics of Golgi structure and function. Our experiments following cargo efflux from the Golgi in single cells revealed the unexpected property of there being no delay in the export of cargo out of the Golgi. Cargo exited the Golgi at a rate that was proportional to its total amount in the Golgi and no cargo was privileged for export based on its time of entry into the Golgi. By constructing our model around a continuous Golgi compartment scheme modified through the inclusion of rapid partitioning between different lipid phases, we show how various properties of the Golgi can be accounted for - including the differential distribution of enzymes and lipids, the exponential cargo efflux kinetics, and the apparent sequential transport of cargo reported in electron micrograph studies. The validity of our model was examined directly by cell-based experiments testing different predictions. The biological responses all fit well with the predictions of the model, providing support to the conclusion that a single-compartment partitioning scheme represents a viable description of the behavior and dynamics of the Golgi apparatus.

Host: Jaakko Saraste, Department for Biomedicine

Dr. Jennifer Lippincott-Schwartz graduated from Swarthmore College in 1974. After several years of teaching high school sciences, both in Kenya and California, she turned to laboratory research working first with Dr. Phil Hanwalt at Stanford University. She then entered the Ph.D. Program at Johns Hopkins University, where her work in the laboratory of Dr. Doug Fambrough revealed that lysosomal membrane proteins lead an itinerant life as they repeatedly cycle to the cell surface and back, although the lysosome itself serves to degrade cellular components. Following award of her doctoral degree in 1986, she continued her focus on membrane trafficking in the laboratory of Dr. Richard Klausner at National Institutes of Health (NIH), where she first identified a nonlysosomal pathway for degradation of proteins in the endoplasmic reticulum (ER), and then demonstrated a retrograde transport pathway for recycling of proteins from the Golgi apparatus to the ER. In 1992, she became Head of the Unit on Organelle Biology in the Cell Biology and Metabolism Branch of the NICHD at NIH. Studying the morphological and kinetic properties of secretory protein transport and the pathways for breakdown and reassembly of the nuclear envelope and Golgi apparatus during mitosis, her laboratory has pioneered the use of green fluorescent protein (GFP) tags for visualisation of membrane protein traffic within cells.

Jennifer Lippincott-Schwartz was recently named as the first Keith Porter Fellow, in honor of the late Keith Porter's achievements in cell biology, she was awarded the Wellcome Visiting Professorship and she serves as a member of the Council for the American Society of Cell Biology.