Structural Basis of Cyclic Nucleotide Selectivity in cGMP-dependent Protein Kinase II

Membrane-bound cGMP-dependent protein kinase (PKG) II is a key regulator of bone growth, renin secretion, and memory formation. Despite its crucial physiological roles, little is known about its cyclic nucleotide selectivity mechanism due to a lack of structural information. Here, we find that the C-terminal cyclic nucleotide binding (CNB-B) domain of PKG II binds cGMP with higher affinity and selectivity when compared with its N-terminal CNB (CNB-A) domain. To understand the structural basis of cGMP selectivity, we solved co-crystal structures of the CNB domains with cyclic nucleotides. Our structures combined with mutagenesis demonstrate that the guanine-specific contacts at Asp-412 and Arg-415 of the αC-helix of CNB-B are crucial for cGMP selectivity and activation of PKG II. Structural comparison with the cGMP selective CNB domains of human PKG I and Plasmodium falciparum PKG (PfPKG) shows different contacts with the guanine moiety, revealing a unique cGMP selectivity mechanism for PKG II.     

Discovery of novel drug targets and their functions using phenotypic screening of natural products

Natural products are valuable resources that provide a variety of bioactive compounds and natural pharmacophores in modern drug discovery. Discovery of biologically active natural products and unraveling their target proteins to understand their mode of action have always been critical hurdles for their development into clinical drugs. For effective discovery and development of bioactive natural products into novel therapeutic drugs, comprehensive screening and identification of target proteins are indispensable. In this review, a systematic approach to understanding the mode of action of natural products isolated using phenotypic screening involving chemical proteomics-based target identification is introduced. This review highlights three natural products recently discovered via phenotypic screening, namely glucopiericidin A, ecumicin, and terpestacin, as representative case studies to revisit the pivotal role of natural products as powerful tools in discovering the novel functions and druggability of targets in biological systems and pathological diseases of interest.     

Heterologous expression of an RNA-binding protein affects flowering time as well as other developmental processes in <Emphasis Type="Italic">Solanaceae</Emphasis>

Flowering time in members of the Solanaceae plant family, such as pepper (Capsicum spp.) and tomato (Solanum lycopersicum), is an important agronomic trait for controlling shoot architecture and improving yield. To investigate the feasibility of flowering time regulation in tomato, an RNA-binding protein (RBP) encoding gene homologous to human Nucleolar protein interacting with the forkhead-associated (FHA) domain of pKI-67 (NIFK), CaRBP, was isolated from hot pepper. The function of CaRBP was determined in transgenic tomato. The deduced amino acid sequence includes an RNA recognition motif (RRM) and showed most similarity to the RRM present in a putative RBP encoded by human NIFK. CaRBP was highly expressed in the vegetative and reproductive tissues, such as leaves and fruits, respectively. Subcellular localization analysis indicated that CaRBP is a nucleolar protein. Heterologous expression of CaRBP under 35S promoter in tomato plants induced severe alteration of flowering with additional defects of vegetative organs. This floral retardation was associated with the alteration of SFT/SP3D and SlSOC1s as floral integrators. Furthermore, CaRBP reduces the expression levels of SlCOLs/TCOLs via changes in the expression of SlCDF3, SlFBHs, and SlFKF1s. This indicates a repressive effect of CaRBP on the regulation of flowering time in tomato. Overall, these results suggest that alteration in CaRBP expression levels may provide an effective means of controlling flowering time in day-neutral Solanaceae.     

Monomerization and ER Relocalization of GRASP Is a Requisite for Unconventional Secretion of CFTR

Induction of endoplasmic reticulum (ER)‐to‐Golgi blockade or ER stress induces Golgi reassembly stacking protein (GRASP)‐mediated, Golgi‐independent unconventional cell‐surface trafficking of the folding‐deficient ΔF508‐cystic fibrosis transmembrane conductance regulator (CFTR). However, molecular mechanisms underlying this process remain elusive. Here, we show that phosphorylation‐dependent dissociation of GRASP homotypic complexes and subsequent relocalization of GRASP to the ER play a critical role in the unconventional secretion of CFTR. Immunolocalization analyses of mammalian cells revealed that the Golgi protein GRASP55 was redistributed to the ER by stimuli that induce unconventional secretion of ΔF508‐CFTR, such as induction of ER‐to‐Golgi blockade by the Arf1 mutant. Notably, the same stimuli also induced phosphorylation of regions near the C‐terminus of GRASP55 and dissociation of GRASP homomultimer complexes. Furthermore, phosphorylation‐mimicking mutations of GRASP55 induced the monomerization and ER relocalization of GRASP55, and these changes were nullified by phosphorylation‐inhibiting mutations. These results provide mechanistic insights into how GRASP accesses the ER‐retained ΔF508‐CFTR and mediates the ER stress‐induced unconventional secretion pathway.      

Multisite phosphorylation of adipocyte and hepatocyte phosphodiesterase 3B

Phosphodiesterase 3B (PDE3B) is an important component of insulin and cAMP-dependent signalling pathways. In order to study phosphorylation of PDE3B, we have used an adenoviral system to express recombinant flag-tagged PDE3B in primary rat adipocytes and H4IIE hepatoma cells. Phosphorylation of PDE3B after treatment of cells with insulin, cAMP-increasing agents, or the phosphatase inhibitor, calyculin A was analyzed by two-dimensional tryptic phosphopeptide mapping and mass spectrometry. We found that PDE3B is multisite phosphorylated in adipocytes and H4IIE hepatoma cells in response to all these stimuli. Several sites were identified; serine (S)273, S296, S421, S424/5, S474 and S536 were phosphorylated in adipocyte as well as H4IIE hepatoma cells whereas S277 and S507 were phosphorylated in hepatoma cells only. Several of the sites were phosphorylated by insulin as well as cAMP-increasing hormones indicating integration of the two signalling pathways upstream of PDE3B, maybe at the level of protein kinase B.