Faculty Bibliography

Glucose-regulated GRP58 has shown clinical applications to endoplasmic reticulum (ER) stress and cancer. GRP58 is localized in the cytosol, endoplasmic reticulum (ER) and nucleus. Twenty-four amino acids at the N-terminal hydrophobic region are known to target GRP58 to ER for synthesis at the ER membrane and translocation into the ER lumen. In addition, GRP58 contains putative nuclear localization (494KPKKKKK500) and ER retention (502QEDL505) signals. However, the role of these signals in nuclear import and ER retention of GRP58 remains unknown. Present studies investigated the signals that control nuclear localization and ER retention of GRP58. Deletion/mutation of nuclear localization signal (NLS) abrogated nuclear import of GRP58. NLS attached to EGFP localized EGFP in the nucleus. However, deletion/mutation of putative ER retention signal alone did not alter ER retention of GRP58. Interestingly, a combined deletion/mutation of NLS and ER retention signals blocked the GRP58 retention in the ER. These results concluded that overlapping NLS and ER retention signal sequences regulate nuclear localization and ER retention of GRP58.

ATP drives the conformational change of the group II chaperonin from the open lid substrate-binding conformation to the closed lid conformation to encapsulate an unfolded protein in the central cavity. The detailed mechanism of this conformational change remains unknown. To elucidate the intra-ring cooperative action of subunits for the conformational change, we constructed Thermococcus chaperonin complexes containing mutant subunits in an ordered manner and examined their folding and conformational change abilities. Chaperonin complexes containing wild-type subunits and mutant subunits with impaired ATP-dependent conformational change ability or ATP hydrolysis activity, one by one, exhibited high protein refolding ability. The effects of the mutant subunits correlate with the number and order in the ring. In contrast, the use of a mutant lacking helical protrusion severely affected the function. Interestingly, these mutant chaperonin complexes also exhibited ATP-dependent conformational changes as demonstrated by small angle x-ray scattering, protease digestion, and changes in fluorescence of the fluorophore attached to the tip of the helical protrusion. However, their conformational change is likely to be transient. They captured denatured proteins even in the presence of ATP, whereas addition of ATP impaired the ability of the wild-type chaperonin to protect citrate synthase from thermal aggregation. These results suggest that ATP binding/hydrolysis causes the independent conformational change of the subunit, and further conformational change for the complete closure of the lid is induced and stabilized by the interaction between helical protrusions.

The bioactive lipid sphingosine 1-phosphate (S1P) and its G protein-coupled receptors play critical roles in cardiovascular, immunological, and neural development and function. Despite its importance, many questions remain about S1P signaling, including how S1P, which is synthesized intracellularly, is released from cells. Mutations in the zebrafish gene encoding the S1P receptor Miles Apart (Mil)/S1P(2) disrupt the formation of the primitive heart tube. We find that mutations of another zebrafish locus, two of hearts (toh), cause phenotypes that are morphologically indistinguishable from those seen in mil/s1p2 mutants. Positional cloning of toh reveals that it encodes a member of the Spinster-like family of putative transmembrane transporters. The biological functions of these proteins are poorly understood, although phenotypes of the Drosophila spinster and zebrafish not really started mutants suggest that these proteins may play a role in lipid trafficking. Through gain- and loss-of-function analyses, we show that toh is required for signaling by S1P(2). Further evidence indicates that Toh is involved in the trafficking or cellular release of S1P.

The relationship between intermembrane spacing, adhesion efficiency, and lateral organization of adhesion receptors has not been established for any adhesion system. We have utilized the CD2 ligand CD48 with two (wild type CD48 (CD48-WT)), four (CD48-CD2), or five (CD48-CD22) Ig-like domains. CD48-WT was 10-fold more efficient in mediating adhesion than CD48-CD2 or CD48-CD22. Electron tomography of contact areas with planar bilayers demonstrated average intermembrane spacing of 12.8 nm with CD48-WT, 14.7 nm with CD48-CD2, and 15.6 nm with CD48-CD22. Both CD48-CD2 and CD48-CD22 chimeras segregated completely from CD48-WT in mixed contact areas. In contrast, CD48-CD2 and CD48-CD22 co-localized when mixed contacts were formed. Confocal imaging of immunological synapses formed between primary T lymphocytes and Chinese hamster ovary cells presenting major histocompatibility complex-peptide complexes, and different forms of CD48 demonstrated that CD48-CD2 and CD48-CD22 induce an eccentric CD2/T cell antigen receptor cluster. We propose that this reorganization of the immunological synapse sequesters the T cell antigen receptor in a location where it cannot interact with its ligand and dramatically reduces T cell sensitivity

Transforming growth factor-beta (TGF-beta) activity is controlled at many levels including the conversion of the latent secreted form to its active state. TGF-beta is often released as part of an inactive tripartite complex consisting of TGF-beta, the TGF-beta propeptide, and a molecule of latent TGF-beta binding protein (LTBP). The interaction of TGF-beta and its cleaved propeptide renders the growth factor latent, and the liberation of TGF-beta from this state is crucial for signaling. To examine the contribution of LTBP to TGF-beta function, we generated mice in which the cysteines that link the propeptide to LTBP were mutated to serines, thereby blocking covalent association. Tgfb1(C33S/C33S) mice had multiorgan inflammation, lack of skin Langerhans cells (LC), and a shortened lifespan, consistent with decreased TGF-beta1 levels. However, the inflammatory response and decreased lifespan were not as severe as observed with Tgfb1(-/-) animals. Tgfb1(C33S/C33S) mice exhibited decreased levels of active TGF-beta1, decreased TGF-beta signaling, and tumors of the stomach, rectum, and anus. These data suggest that the association of LTBP with the latent TGF-beta complex is important for proper TGF-beta1 function and that Tgfb1(C33S/C33S) mice are hypomorphs for active TGF-beta1. Moreover, although mechanisms exist to activate latent TGF-beta1 in the absence of LTBP, these mechanisms are not as efficient as those that use the latent complex containing LTBP