Juan Bonifacino, PhD, Chief

The Cell Biology and Metabolism Branch (CBMB) conducts studies in various areas of molecular cell biology, including the mechanisms of intracellular protein trafficking and organelle biogenesis, iron metabolism, the adaptive responses to environmental stresses, and the regulation of the cell cycle during oogenesis. A salient feature of the CBMB is its outstanding capabilities in state-of-the-art fluorescence microscopy techniques, which permit cell imaging in real time, photobleaching, fluorescence resonance energy transfer, and fluorescence correlation spectroscopy. In addition, the CBMB maintains facilities for working with several organisms, including bacteria, yeast, Drosophila, mice, and mammalian cells. The Branch applies knowledge gained from the study of basic cellular processes to elucidating the causes of human diseases, including neurodegeneration, viral pathogenesis and disorders of protein trafficking, iron overload, and bile acid secretion.

Over the past year, the Unit on Cellular Polarity, headed by Irwin Arias, has continued its studies on the mechanisms responsible for the selective trafficking of proteins to the apical domain of hepatocytes and other polarized cells. The goal of the studies is to identify the components and regulation of these mechanisms, their role in creating and maintaining cellular polarity, and the molecular defects responsible for inheritable and acquired bile secretory failure (cholestasis).

The Section on Intracellular Protein Trafficking, led by Juan Bonifacino, has discovered and characterized critical components of the molecular machinery responsible for cell compartmentalization and exchange of materials between cells and their environment. Among the novel components identified by the Section are a protein that is defective in a mouse model of Hermansky-Pudlak syndrome, a pigmentation and bleeding disorder. In addition, the group has elucidated the roles of various adaptor proteins in sorting to lysosomes, antigen-processing compartments, and melanosomes.

Ramanujan Hegde’s Unit on Protein Biogenesis has discovered a new site of cellular regulation, namely, the translocation of secretory and membrane proteins into the mammalian endoplasmic reticulum. In one particularly noteworthy example, the Unit found that the translocation of some proteins into the ER was selectively reduced during certain types of stress. This protective pathway, termed pre-emptive quality control, attenuates the adverse consequences of protein misfolding in the ER. The Unit is now examining the importance of this novel pathway for proteins associated with neurodegenerative diseases.

Catherine Jackson’s Unit on GTPase Regulation of Membrane Traffic has identified a new role for an activator (Gea2p) of the Arf1 GTPase. The group has shown that Gea2p is required for forward trafficking through the Golgi without affecting several retrograde trafficking pathways. It is currently believed that forward transport of cargo through the Golgi is accomplished by default as a result of retrograde trafficking of Golgi modification enzymes. Hence, the Unit’s recent results call into question the current model and suggest that the Arf GEFs are directly involved in forward transport.

Mary Lilly’s Unit on Cell Cycle Regulation studies cell cycle regulation during gametogenesis. Over the past year, the Unit has shown that the translational inhibitor Bruno is an integral player in coordinating cell cycle regulation and gamete differentiation during oogenesis. In addition, the group found that the cyclin-dependent kinase inhibitor p27/Dacapo promotes the licensing of DNA replication origins during the endocycle. The work has important implications for the role of cyclin-dependent kinase inhibitors in the maintenance of genomic stability in both mitotic and endocycling cells.

Jennifer Lippincott-Schwartz, who heads the Section on Organelle Biology, has continued to use novel fluorescence imaging tools to investigate many important cellular processes. The following are among the new findings from her group: (1) the breakdown and reassembly of the Golgi apparatus involves sequential inactivation and reactivation of the small GTPases Arf1 and Sar1; (2) Golgi membrane proteins redistribute to the ER or to ER exit sites during mitosis; (3) ArfGAP1 is a bona fide component of the coat lattice that regulates the dynamics of the lattice independently of coated vesicle production; and (4) ER and Golgi membranes exist as restricted and compartmentalized units that surround individual nuclei in the Drosophila syncitial blastoderm embryo.

The Section on Human Iron Metabolism, headed by Tracey Rouault, studies the regulation of iron metabolism. The group discovered that an iron exporter known as ferroportin is present in synaptic vesicles and that the iron storage protein ferritin is transported into neuronal axons. Unusual iron distribution and trafficking patterns may explain why neurons are unusually susceptible to damage when iron metabolism is misregulated. The discoveries may promote understanding of neurodegenerative diseases, especially Parkinson’s disease.

Gisela Storz’s Section on Environmental Gene Regulation has continued its studies on the mechanisms by which cells defend against oxidative stress. Recent work focused on structural studies of key regulators of the cellular responses to hydrogen peroxide in bacteria and yeast. The group has also continued its identification and characterization of untranslated, regulatory RNAs. Systematic screens have led to the identification of more than 60 new non-coding RNAs in E. coli whose functions are under study. Many of these bacterial regulatory RNAs act analogously to eukaryotic miRNA and siRNAs to regulate mRNA stability and translation.