Mechanistic roles of tyrosine phosphorylation in reversible amyloids, autoinhibition, and endosomal membrane association of ALIX.

Human apoptosis-linked gene-2 interacting protein X (ALIX), a versatile adapter protein, regulates essential cellular processes by shuttling between late endosomal membranes and the cytosol, determined by its interactions with Src kinase. Here, we investigate the molecular basis of these transitions and the effects of tyrosine phosphorylation on the interplay between structure, assembly, and intramolecular and intermolecular interactions of ALIX. As evidenced by transmission electron microscopy, fluorescence and circular dichroism spectroscopy, the proline-rich domain of ALIX, which encodes binding epitopes of multiple cellular partners, formed rope-like ß-sheet-rich reversible amyloid fibrils that dissolved upon Src-mediated phosphorylation and were restored on protein-tyrosine phosphatase 1B-mediated dephosphorylation of its conserved tyrosine residues. Analyses of the Bro1 domain of ALIX by solution NMR spectroscopy elucidated the conformational changes originating from its phosphorylation by Src and established that Bro1 binds to hyperphosphorylated proline-rich domain and to analogs of late endosomal membranes via its highly basic surface. These results uncover the autoinhibition mechanism that relocates ALIX to the cytosol and the diverse roles played by tyrosine phosphorylation in cellular and membrane functions of ALIX.

Quantitative NMR Study of Insulin-Degrading Enzyme Using Amyloid-ß and HIV-1 p6 Elucidates Its Chaperone Activity.

Insulin-degrading enzyme (IDE) hydrolyzes monomeric polypeptides, including amyloid-ß (Aß) and HIV-1 p6. It also acts as a nonproteolytic chaperone to prevent Aß polymerization. Here we compare interactions of Aß and non-amyloidogenic p6 with IDE. Although both exhibited similar proteolysis rates, the binding kinetics to an inactive IDE characterized using relaxation-based NMR were remarkably different. IDE and Aß formed a sparsely populated complex with a lifetime of milliseconds in which a short hydrophobic cleavage segment of Aß was anchored to IDE. Strikingly, a second and more stable complex was significantly populated with a subsecond lifetime owing to multiple intermolecular contacts between Aß and IDE. By selectively sequestering Aß in this nonproductive complex, IDE likely increases the critical concentration required for fibrillization. In contrast, IDE and p6 formed a transient, submillisecond complex involving a single anchoring p6 motif. Modulation of intermolecular interactions, thus, allows IDE to differentiate between non-amyloidogenic and amyloidogenic substrates.

Bacterial production of site specific 13C labeled phenylalanine and methodology for high level incorporation into bacterially expressed recombinant proteins.

Nuclear magnetic resonance spectroscopy studies of ever larger systems have benefited from many different forms of isotope labeling, in particular, site specific isotopic labeling. Site specific 13C labeling of methyl groups has become an established means of probing systems not amenable to traditional methodology. However useful, methyl reporter sites can be limited in number and/or location. Therefore, new complementary site specific isotope labeling strategies are valuable. Aromatic amino acids make excellent probes since they are often found at important interaction interfaces and play significant structural roles. Aromatic side chains have many of the same advantages as methyl containing amino acids including distinct 13C chemical shifts and multiple magnetically equivalent 1H positions. Herein we report economical bacterial production and one-step purification of phenylalanine with 13C incorporation at the Cα, Cγ and Cε positions, resulting in two isolated 1H-13C spin systems. We also present methodology to maximize incorporation of phenylalanine into recombinantly overexpressed proteins in bacteria and demonstrate compatibility with ILV-methyl labeling. Inexpensive, site specific isotope labeled phenylalanine adds another dimension to biomolecular NMR, opening new avenues of study.