In-cell NMR is an isotope-aided multi-dimensional NMR technique that enables observations of conformations and functions of proteins in living cells at the atomic level. This method has been successfully applied to proteins overexpressed in bacteria, providing information on protein-ligand interactions and conformations. However, the application of in-cell NMR to eukaryotic cells has been limited to Xenopus laevis oocytes. Wider application of the technique is hampered by inefficient delivery of isotope-labelled proteins into eukaryote somatic cells. Here we describe a method to obtain high-resolution two-dimensional (2D) heteronuclear NMR spectra of proteins inside living human cells. Proteins were delivered to the cytosol by the pyrenebutyrate-mediated action of cell-penetrating peptides linked covalently to the proteins. The proteins were subsequently released from cell-penetrating peptides by endogenous enzymatic activity or by autonomous reductive cleavage. The heteronuclear 2D spectra of three different proteins inside human cells demonstrate the broad application of this technique to studying interactions and protein processing. The in-cell NMR spectra of FKBP12 (also known as FKBP1A) show the formation of specific complexes between the protein and extracellularly administered immunosuppressants, demonstrating the utility of this technique in drug screening programs. Moreover, in-cell NMR spectroscopy demonstrates that ubiquitin has much higher hydrogen exchange rates in the intracellular environment, possibly due to multiple interactions with endogenous proteins.

DNA methylation of CpG dinucleotides is an important epigenetic modification of mammalian genomes and is essential for the regulation of chromatin structure, of gene expression and of genome stability. Differences in DNA methylation patterns underlie a wide range of biological processes, such as genomic imprinting, inactivation of the X chromosome, embryogenesis, and carcinogenesis. Inheritance of the epigenetic methylation pattern is mediated by the enzyme DNA methyltransferase 1 (Dnmt1), which methylates newly synthesized CpG sequences during DNA replication, depending on the methylation status of the template strands. The protein UHRF1 (also known as Np95 and ICBP90) recognizes hemi-methylation sites via a SET and RING-associated (SRA) domain and directs Dnmt1 to these sites. Here we report the crystal structures of the SRA domain in free and hemi-methylated DNA-bound states. The SRA domain folds into a globular structure with a basic concave surface formed by highly conserved residues. Binding of DNA to the concave surface causes a loop and an amino-terminal tail of the SRA domain to fold into DNA interfaces at the major and minor grooves of the methylation site. In contrast to fully methylated CpG sites recognized by the methyl-CpG-binding domain, the methylcytosine base at the hemi-methylated site is flipped out of the DNA helix in the SRA-DNA complex and fits tightly into a protein pocket on the concave surface. The complex structure suggests that the successive flip out of the pre-existing methylated cytosine and the target cytosine to be methylated is associated with the coordinated transfer of the hemi-methylated CpG site from UHRF1 to Dnmt1.

Most of the currently available drugs for osteoporosis inhibit osteoclastic bone resorption; only a few drugs promote osteoblastic bone formation. It is thus becoming increasingly necessary to identify the factors that regulate bone formation. We found that osteoclasts express semaphorin 4D (Sema4D), previously shown to be an axon guidance molecule, which potently inhibits bone formation. The binding of Sema4D to its receptor Plexin-B1 on osteoblasts resulted in the activation of the small GTPase RhoA, which inhibits bone formation by suppressing insulin-like growth factor-1 (IGF-1) signaling and by modulating osteoblast motility. Sema4d-/- mice, Plxnb1-/- mice and mice expressing a dominant-negative RhoA specifically in osteoblasts showed an osteosclerotic phenotype due to augmented bone formation. Notably, Sema4D-specific antibody treatment markedly prevented bone loss in a model of postmenopausal osteoporosis. Thus, Sema4D has emerged as a new therapeutic target for the discovery and development of bone-increasing drugs.