Cell Biology and Metabolism Branch

Cell biology and metabolism branch

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, adaptive responses to environmental stresses, and regulation of the cell cycle during oogenesis. The CBMB’s outstanding microscopy facilities permit the use of state-of-the-art techniques such as fluorescent imaging of cells in real time, photobleaching, fluorescence resonance energy transfer, fluorescence correlation spectroscopy, and image analysis in the study of cell structure and dynamics. In addition, the CBMB is equipped with facilities for work with many model organisms, including bacteria, yeast, Drosophila melanogaster, mice, and mammalian cells. Members of the CBMB apply knowledge gained from the study of basic cell-biological problems to the elucidation of the causes of human diseases, including disorders of lysosome-related organelles, iron overload, and viral pathogenesis.

Over the past year, the Section on Intracellular Protein Trafficking, led by Juan Bonifacino, discovered critical components of the molecular machinery involved in the exchange of materials between cells and their environment. In particular, the group has contributed to the identification of proteins that determine transport of molecules to endosomes and lysosomes. Among the identified components are novel protein complexes that are defective in mouse models of Hermansky-Pudlak syndrome, a pigmentation and bleeding disorder. In addition, the group has discovered that defective protein biogenesis in the endoplasmic reticulum (ER) underlies another genetic disorder, autosomal dominant polycystic liver disease.

Ramanujan Hegde’s group, the Unit on Protein Biogenesis, discovered a novel site of cellular regulation, the translocation of secretory and membrane protein substrates into the mammalian ER. During the past year, the group elucidated several examples of regulated protein translocation that affect both normal physiology and disease progression. In one particularly noteworthy example, the researchers found that the generation of potentially neurotoxic forms of the prion protein was controlled during its translocation into the ER. The consequence of misregulating prion protein translocation for the development of neurodegeneration is now under investigation in transgenic mouse model systems recently developed by the group.

Catherine Jackson’s group, the Unit on GTPase Regulation of Membrane Traffic, identified interacting partners of activators (GEFs) of the Arf GTPase, a central regulator of organelle structure and function in eukaryotic cells. In particular, she has identified the Golgi-localized, vesicle-tethering complex TRAPPII as a direct partner of the Gea2p GEF. TRAPPII is also an effector of Arf; hence the interactions demonstrate coordination of the events of vesicle budding and vesicle fusion within the Golgi apparatus. This and other interactions are shedding new light on the mechanisms of trafficking through the secretory pathway that are conserved throughout eukaryotic evolution.

Mary Lilly’s group, the Unit on Cell Cycle Regulation, studies cell cycle regulation during gametogenesis. Over the last year, the group characterized a highly conserved gene, mio, that provides a link between the response to DNA damage and meiotic progression. In addition, the group has explored how mitotic quiescence is maintained during meiosis and discovered a unique role for translational repression. These studies have provided novel insights into the relationship between early meiotic progression and oocyte development.

Jennifer Lippincott-Schwartz’s group, the Section on Organelle Biology, has continued to use novel fluorescence imaging tools, including multispectral time-lapse imaging, photobleaching, and photoactivation, to investigate many important cellular processes, such as cell division, protein secretion, organelle transformation, and the mechanism for raft formation at the plasma membrane. Her group recently discovered that the behavior of chromosomes, cytoskeleton, and Golgi during mitosis is coordinated through the activity of the small GTPase Arf1; that organelles are capable of transforming from reticular networks into tightly stacked arrays through low-affinity protein-protein interactions; that proteins residing within raft domains at the plasma membrane undergo rapid partitioning in and out of these domains; and that delivery of raft-associated proteins to apical surfaces of polarized cells occurs by an indirect rather than direct route.

Tracey Rouault’s group, the Section on Human Iron Metabolism, 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. These discoveries may promote understanding of neurodegenerative diseases, especially Parkinson’s disease.

Gisela Storz’s group, the Section on Environmental Gene Regulation, has continued its studies on the mechanisms by which cells defend against oxidative stress, including analyses of key regulators of the cellular responses to hydrogen peroxide in bacteria and yeast. Another area of research is the identification and characterization of untranslated, regulatory RNAs. Systematic screens for noncoding RNA genes in E. coli resulted in the identification of more than 30 new noncoding RNAs whose functions are now under investigation.