Biomedical subjects

Tony Chu

Publications and source records attributed to Tony Chu.

4 recordsLinked to original sources

PubMed2006

New component of ESCRT-I regulates endosomal sorting complex assembly.

The endosomal sorting complex required for transport (ESCRT) complexes play a critical role in receptor down-regulation and retroviral budding. Although the crystal structures of two ESCRT complexes have been determined, the molecular mechanisms underlying the assembly and regulation of the ESCRT machinery are still poorly understood. We identify a new component of the ESCRT-I complex, multivesicular body sorting factor of 12 kD (Mvb12), and demonstrate that Mvb12 binds to the coiled-coil domain of the ESCRT-I subunit vacuolar protein sorting 23 (Vps23). We show that ESCRT-I adopts an oligomeric state in the cytosol, the formation of which requires the coiled-coil domain of Vps23, as well as Mvb12. Loss of Mvb12 results in the disassembly of the ESCRT-I oligomer and the formation of a stable complex of ESCRT-I and -II in the cytosol. We propose that Mvb12 stabilizes ESCRT-I in an oligomeric, inactive state in the cytosol to ensure that the ordered recruitment and assembly of ESCRT-I and -II is spatially and temporally restricted to the surface of the endosome after activation of the MVB sorting reaction.

Cell Compartmentation↗

PubMed2005

Cobalt catalysts for the alternating copolymerization of propylene oxide and carbon dioxide: combining high activity and selectivity.

Synthetic pathways to (salcy)CoX (salcy = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-diaminocyclohexane; X = halide or carboxylate) complexes are described. Complexes (R,R)-(salcy)CoCl, (R,R)-(salcy)CoBr, (R,R)-(salcy)CoOAc, and (R,R)-(salcy)CoOBzF(5) (OBzF(5) = pentafluorobenzoate) are highly active catalysts for the living, alternating copolymerization of propylene oxide (PO) and CO(2), yielding poly(propylene carbonate) (PPC) with no detectable byproducts. The PPC generated using these catalyst systems is highly regioregular and has up to 99% carbonate linkages with a narrow molecular weight distribution (MWD). Inclusion of the cocatalysts [PPN]Cl or [PPN][OBzF(5)] ([PPN] = bis(triphenylphosphine)iminium) with complex (R,R)-(salcy)CoCl, (R,R)-(salcy)CoBr, or (R,R)-(salcy)CoOBzF(5) results in remarkable activity enhancement of the copolymerization as well as improved stereoselectivity and regioselectivity with maximized reactivity at low CO(2) pressures. In the case of [PPN]Cl with (R,R)-(salcy)CoOBzF(5), an unprecedented catalytic activity of 620 turnovers per hour is achieved for the copolymerization of rac-PO and CO(2), yielding iso-enriched PPC with 94% head-to-tail connectivity. The stereochemistry of the monomer and catalyst used in the copolymerization has dramatic effects on catalytic activity and the PPC microstructure. Using catalyst (R,R)-(salcy)CoBr with (S)-PO/CO(2) generates highly regioregular PPC, whereas using (R)-PO/CO(2) with the same catalyst gives an almost completely regiorandom copolymer. The rac-PO/CO(2) copolymerization with catalyst rac-(salcy)CoBr yields syndio-enriched PPC, an unreported PPC microstructure. In addition, (R,R)-(salcy)CoOBzF(5)/[PPN]Cl copolymerizes (S)-PO and CO(2) with a turnover frequency of 1100 h(-1), an activity surpassing that observed in any previously reported system.

Journal Article↗

PubMed2003

Multivesicular body sorting: ubiquitin ligase Rsp5 is required for the modification and sorting of carboxypeptidase S.

The multivesicular body (MVB) sorting pathway provides a mechanism for delivering transmembrane proteins into the lumen of the lysosome/vacuole. Recent studies demonstrated that ubiquitin modification acts in cis as a signal for the sorting of cargoes into this pathway. Here, we present results from a genetic selection designed to identify mutants that missort MVB cargoes. This selection identified a point mutation in ubiquitin ligase Rsp5 (Rsp5-326). At the permissive temperature, this mutant is specifically defective for ubiquitination and sorting of the ubiquitin-dependent MVB cargo precursor carboxypeptidase S (pCPS), but not ligand-induced ubiquitination of Ste2. A previous study implicated Tul1 as the ubiquitin ligase responsible for MVB sorting of pCPS. However, we detected no defect in either the sorting or ubiquitination of pCPS in tul1 mutants. We had previously shown that Fab1 phosphatidylinositol 3-phosphate 5-kinase is also required for MVB sorting of pCPS, but not Ste2. However, our analyses reveal that fab1 mutants do not exhibit a defect in ubiquitination of pCPS. Thus, both Rsp5 and Fab1 play distinct and essential roles in the targeting of biosynthetic MVB cargoes. However, whereas Rsp5 seems to be responsible for cargo ubiquitination, the precise role for Fab1 remains to be elucidated.

Carboxypeptidases↗

PubMed2002

Gas channels for NH(3): proteins from hyperthermophiles complement an Escherichia coli mutant.

Ammonium transport (Amt) proteins appear to be bidirectional channels for NH(3). The amt genes of the hyperthermophiles Aquifex aeolicus and Methanococcus jannaschii complement enteric amtB mutants for growth at 25 nM NH(3) at 37 degrees C. To our knowledge, Amt proteins are the first hyperthermophilic membrane transport proteins shown to be active in a mesophilic bacterium. Despite low expression levels, His-tagged Aquifex Amt could be purified by heating and nickel chelate affinity chromatography. It could be studied genetically in Escherichia coli.

Ammonia↗