Constitutive activation and oncogenicity are mediated by loss of helical structure at the cytosolic boundary of thrombopoietin receptor mutant dimers.

Dimerization of the thrombopoietin receptor (TpoR) is necessary for receptor activation and downstream signaling through activated Janus kinase 2. We have shown previously that different orientations of the transmembrane (TM) helices within a receptor dimer can lead to different signaling outputs. Here we addressed the structural basis of activation for receptor mutations S505N and W515K that induce myeloproliferative neoplasms. We show using in vivo bone marrow reconstitution experiments that ligand-independent activation of TpoR by TM asparagine (Asn) substitutions is proportional to the proximity of the Asn mutation to the intracellular membrane surface. Solid-state NMR experiments on TM peptides indicate a progressive loss of helical structure in the juxtamembrane (JM) R/KWQFP motif with proximity of Asn substitutions to the cytosolic boundary. Mutational studies in the TpoR cytosolic JM region show that loss of the helical structure in the JM motif by itself can induce activation, but only when localized to a maximum of six amino acids downstream of W515, the helicity of the remaining region until Box 1 being required for receptor function. The constitutive activation of TpoR mutants S505N and W515K can be inhibited by rotation of TM helices within the TpoR dimer, which also restores helicity around W515. Together, these data allow us to develop a general model for activation of TpoR and explain the critical role of the JM W515 residue in the regulation of the activity of the receptor.

Identification of antioxidant peptides derived from tropical jackfruit seed and investigation of the stability profiles.

Jackfruit is a sweet tropical fruit with very pleasant aroma, and the ripe seeds are edible. In this study, jackfruit seed proteins were isolated and subjected to trypsin digestion. The resultant protein hydrolysate was then subjected to antioxidant assay-guided purification, using centrifugal filtration, C18 reverse-phase and strong cation exchange (SCX) fractionations. The purified SCX fraction was further analyzed by de novo peptide sequencing, and two peptide sequences were identified and synthesized. Peptide JFS-2 (VGPWQK) was detected with antioxidant potential, with EC50 value comparable to that of commercial GSH antioxidant peptide. Additionally, the identified peptides were tested with protein protection potential, in an albumin protein denaturation inhibitory assay. Concurrently, we also investigated the pH, temperature, and gastrointestinal-digestion stability profiles for the identified peptide. With further research efforts, the identified peptides could potentially be developed into preservative agent for protein-rich food systems or as health-promoting diet supplements.

Cerebral vascular amyloid seeds drive amyloid ß-protein fibril assembly with a distinct anti-parallel structure.

Cerebrovascular accumulation of amyloid ß-protein (Aß), a condition known as cerebral amyloid angiopathy (CAA), is a common pathological feature of patients with Alzheimer's disease. Familial Aß mutations, such as Dutch-E22Q and Iowa-D23N, can cause severe cerebrovascular accumulation of amyloid that serves as a potent driver of vascular cognitive impairment and dementia. The distinctive features of vascular amyloid that underlie its unique pathological properties remain unknown. Here, we use transgenic mouse models producing CAA mutants (Tg-SwDI) or overproducing human wild-type Aß (Tg2576) to demonstrate that CAA-mutant vascular amyloid influences wild-type Aß deposition in brain. We also show isolated microvascular amyloid seeds from Tg-SwDI mice drive assembly of human wild-type Aß into distinct anti-parallel ß-sheet fibrils. These findings indicate that cerebrovascular amyloid can serve as an effective scaffold to promote rapid assembly and strong deposition of Aß into a unique structure that likely contributes to its distinctive pathology.

His499 Regulates Dimerization and Prevents Oncogenic Activation by Asparagine Mutations of the Human Thrombopoietin Receptor.

Ligand binding to the extracellular domain of the thrombopoietin receptor (TpoR) imparts a specific orientation on the transmembrane (TM) and intracellular domains of the receptors that is required for physiologic activation via receptor dimerization. To map the inactive and active dimeric orientations of the TM helices, we performed asparagine (Asn)-scanning mutagenesis of the TM domains of the murine and human TpoR. Substitution of Asn at only one position (S505N) activated the human receptor, whereas Asn substitutions at several positions activated the murine receptor. Second site mutational studies indicate that His(499) near the N terminus of the TM domain is responsible for protecting the human receptor from activation by Asn mutations. Structural studies reveal that the sequence preceding His(499) is helical in the murine receptor but non-helical in peptides corresponding to the TM domain of the inactive human receptor. The activating S505N mutation and the small molecule agonist eltrombopag both induce helix in this region of the TM domain and are associated with dimerization and activation of the human receptor. Thus, His(499) regulates the activation of human TpoR and provides additional protection against activating mutations, such as oncogenic Asn mutations in the TM domain.