Retinol modulates site-specific mobility of apo-cellular retinol-binding protein to promote ligand binding

A fundamental question in protein science is how the inherent dynamics of a protein influence its function. If this function involves interactions with a ligand, the protein-ligand encounter has the potential to modulate the protein dynamics. This study reveals how site-specific mobility can be modulated by the ligand to facilitate high affinity binding. We have investigated the mechanism of retinol uptake by the cellular retinol-binding protein type I (CRBP) using line shape analysis of NMR signals. The highly similar structures of apo- and holo-CRBP exhibit closed conformations that seemingly offer no access to ligand, yet the protein binds retinol rapidly and with high affinity. NMR line shape analysis reveals how protein dynamics resolve this apparent paradox. An initial non-specific encounter with the ligand induces the formation of long-lived conformers in the portal region of CRBP suggesting a mechanism how retinol accesses the cavity

Metabolic plasticity in CLL: adaptation to the hypoxic niche.

Metabolic transformation in cancer is increasingly well understood. However, little is known about the metabolic responses of cancer cells that permit their survival in different microenvironments. We have used a nuclear magnetic resonance based approach to monitor metabolism in living primary chronic lymphoid leukemia (CLL) cells and to interrogate their real-time metabolic responses to hypoxia. Our studies demonstrate considerable metabolic plasticity in CLL cells. Despite being in oxygenated blood, circulating CLL cells are primed for hypoxia as measured by constitutively low level hypoxia-inducible factor (HIF-1α) activity and modest lactate production from glycolysis. Upon entry to hypoxia we observed rapid upregulation of metabolic rates. CLL cells that had adapted to hypoxia returned to the 'primed' state when re-oxygenated and again showed the same adaptive response upon secondary exposure to hypoxia. We also observed HIF-1α independent differential utilization of pyruvate in oxygenated and hypoxic conditions. When oxygenated, CLL cells released pyruvate, but in hypoxia imported pyruvate to protect against hypoxia-associated oxidative stress. Finally, we identified a marked association of slower resting glucose and glutamine consumption, and lower alanine and lactate production with Binet A0 stage samples indicating that CLL may be divided into tumors with higher and lower metabolic states that reflect disease stage

Structural basis for a novel intrapeptidyl H-bond and reverse binding of c-Cbl-TKB domain substrates

The c-Cbl tyrosine kinase binding domain (Cbl-TKB), essentially an ‘embedded' SH2 domain, has a critical role in targeting proteins for ubiquitination. To address how this domain can bind to disparate recognition mofits and to determine whether this results in variations in substrate-binding affinity, we compared crystal structures of the Cbl-TKB domain complexed with phosphorylated peptides of Sprouty2, Sprouty4, epidermal growth factor receptor, Syk, and c-Met receptors and validated the binding with point-mutational analyses using full-length proteins. An obligatory, intrapeptidyl H-bond between the phosphotyrosine and the conserved asparagine or adjacent arginine is essential for binding and orientates the peptide into a positively charged pocket on c-Cbl. Surprisingly, c-Met bound to Cbl in the reverse direction, which is unprecedented for SH2 domain binding. The necessity of this intrapeptidyl H-bond was confirmed with isothermal titration calorimetry experiments that also showed Sprouty2 to have the highest binding affinity to c-Cbl; this may enable the selective sequestration of c-Cbl from other target proteins