New insights into the phosphorylation of the threonine motif of the β1 integrin cytoplasmic domain

Integrins require an activation step before ligand binding and signaling that is mediated by talin and kindlin binding to the β integrin cytosolic domain (β-tail). Conflicting reports exist about the contribution of phosphorylation of a conserved threonine motif in the β1-tail (β1-pT788/pT789) to integrin activation. We show that widely used and commercially available antibodies against β1-pT788/pT789 integrin do not detect specific β1-pT788/pT789 integrin signals in immunoblots of several human and mouse cell lysates but bind bi-phosphorylated threonine residues in numerous proteins, which were identified by mass spectrometry experiments. Furthermore, we found that fibroblasts and epithelial cells expressing the phospho-mimicking β1-TT788/789DD integrin failed to activate β1 integrins and displayed reduced integrin ligand binding, adhesion initiation and cell spreading. These cellular defects are specifically caused by the inability of kindlin to bind β1-tail polypeptides carrying a phosphorylated threonine motif or phospho-mimicking TT788/789DD substitutions. Our findings indicate that the double-threonine motif in β1-class integrins is not a major phosphorylation site but if phosphorylated would curb integrin function.

A pivotal role of serine 225 phosphorylation in the function of hepatitis C virus NS5A revealed with the application of a phosphopeptide antiserum and super-resolution microscopy

NS5A is a multi-functional phosphoprotein that plays a key role in both viral replication and assembly. The identity of the kinases that phosphorylate NS5A, and the consequences for HCV biology, remain largely undefined. We previously identified serine 225 (S225) within low complexity sequence (LCS) I as a major phosphorylation site and used a phosphoablatant mutant (S225A) to define a role for S225 phosphorylation in the regulation of genome replication, interactions of NS5A with several host proteins and the sub-cellular localisation of NS5A. To investigate this further, we raised an antiserum to S225 phosphorylated NS5A (pS225). Western blot analysis revealed that pS225 was exclusively found in the hyper-phosphorylated NS5A species. Furthermore, using kinase inhibitors we demonstrated that S225 was phosphorylated by casein kinase 1α (CK1α) and polo-like kinase 1 (PLK1). Using a panel of phosphoablatant mutants of other phosphorylation sites in LCSI we obtained the first direct evidence of bidirectional hierarchical phosphorylation initiated by phosphorylation at S225.Using super-resolution microscopy (Airyscan and Expansion), we revealed a unique architecture of NS5A-positive clusters in HCV-infected cells - pS225 was concentrated on the surface of these clusters, close to lipid droplets. Pharmacological inhibition of S225 phosphorylation resulted in the condensation of NS5A-positive clusters into larger structures, recapitulating the S225A phenotype. Although S225 phosphorylation was not specifically affected by daclatasvir treatment, the latter also resulted in a similar condensation. These data are consistent with a key role for S225 phosphorylation in the regulation of NS5A function.ImportanceNS5A has obligatory roles in the hepatitis C virus lifecycle, and is proposed to be regulated by phosphorylation. As NS5A is a target for highly effective direct-acting antivirals (DAAs) such as daclatasvir (DCV) it is vital to understand how phosphorylation occurs and regulates NS5A function. We previously identified serine 225 (S225) as a major phosphorylation site. Here we used an antiserum specific for NS5A phosphorylated at S225 (pS225-NS5A) to identify which kinases phosphorylate this residue. Using super-resolution microscopy we showed that pS225 was present in foci on the surface of larger NS5A-positive clusters likely representing genome replication complexes. This location would enable pS225-NS5A to interact with cellular proteins and regulate the function and distribution of these complexes. Both loss of pS225 and DCV treatment resulted in similar changes to the structure of these complexes, suggesting that DAA treatment might target a function of NS5A that is also regulated by phosphorylation.

Roles of Phosphorylation of the Nucleocapsid Protein of Mumps Virus in Regulating Viral RNA Transcription and Replication

ABSTRACTMumps virus (MuV) is a paramyxovirus with a negative-sense nonsegmented RNA genome. The viral RNA genome is encapsidated by the nucleocapsid protein (NP) to form the ribonucleoprotein (RNP), which serves as a template for transcription and replication. In this study, we investigated the roles of phosphorylation sites of NP in MuV RNA synthesis. Using radioactive labeling, we first demonstrated that NP was phosphorylated in MuV-infected cells. Using both liquid chromatography-mass spectrometry (LC-MS) andin silicomodeling, we identified nine putative phosphorylated residues within NP. We mutated these nine residues to alanine. Mutation of the serine residue at position 439 to alanine (S439A) was found to reduce the phosphorylation of NP in transfected cells by over 90%. The effects of these mutations on the MuV minigenome system were examined. The S439A mutant was found to have higher activity, four mutants had lower activity, and four mutants had similar activity compared to wild-type NP. MuV containing the S439A mutation had 90% reduced phosphorylation of NP and enhanced viral RNA synthesis and viral protein expression at early time points after infection, indicating that S439 is the major phosphorylation site of NP and its phosphorylation plays an important role in downregulating viral RNA synthesis.IMPORTANCEMumps virus (MuV), a paramyxovirus, is an important human pathogen that is reemerging in human populations. Nucleocapsid protein (NP) of MuV is essential for viral RNA synthesis. We have identified the major phosphorylation site of NP. We have found that phosphorylation of NP plays a critical role in regulating viral RNA synthesis. The work will lead to a better understanding of viral RNA synthesis and possible novel targets for antiviral drug development.

ROCKII Ser1366 phosphorylation reflects the activation status

ROCK (Rho-associated protein kinase), a downstream effector of RhoA, plays an important role in many cellular processes. Accumulating evidence has shown the involvement of ROCK activation in the pathogenesis of many diseases. However, a reagent capable of detecting ROCK activation directly is lacking. In the present study, we show autophosphorylation of ROCKII in an in vitro kinase reaction. The phosphorylation sites were identified by MS, and the major phosphorylation site was found to be at the highly conserved residue Ser1366. A phospho-specific antibody was generated that can specifically recognize ROCKII Ser1366 phosphorylation. We found that the extent of Ser1366 phosphorylation of endogenous ROCKII is correlated with that of myosin light chain phosphorylation in cells in response to RhoA stimulation, showing that Ser1366 phosphorylation reflects its kinase activity. In addition, ROCKII Ser1366 phosphorylation could be detected in human breast tumours by immunohistochemical staining. The present study provides a new approach for revealing the ROCKII activation status by probing ROCKII Ser1366 phosphorylation directly in cells or tissues.

Activation of Pre-mRNA Splicing by Human RNPS1 Is Regulated by CK2 Phosphorylation

ABSTRACT Human RNPS1 was originally characterized as a pre-mRNA splicing activator in vitro and was shown to regulate alternative splicing in vivo. RNPS1 was also identified as a protein component of the splicing-dependent mRNP complex, or exon-exon junction complex (EJC), and a role for RNPS1 in postsplicing processes has been proposed. Here we demonstrate that RNPS1 incorporates into active spliceosomes, enhances the formation of the ATP-dependent A complex, and promotes the generation of both intermediate and final spliced products. RNPS1 is phosphorylated in vivo and interacts with the CK2 (casein kinase II) protein kinase. Serine 53 (Ser-53) of RNPS1 was identified as the major phosphorylation site for CK2 in vitro, and the same site is also phosphorylated in vivo. The phosphorylation status of Ser-53 significantly affects splicing activation in vitro, but it does not perturb the nuclear localization of RNPS1. In vivo experiments indicated that the phosphorylation of RNPS1 at Ser-53 influences the efficiencies of both splicing and translation. We propose that RNPS1 is a splicing regulator whose activator function is controlled in part by CK2 phosphorylation.

Phosphorylation of septin 3 on Ser-91 by cGMP-dependent protein kinase-I in nerve terminals

The septins are a family of GTPase enzymes required for cytokinesis and play a role in exocytosis. Among the ten vertebrate septins, Sept5 (CDCrel-1) and Sept3 (G-septin) are primarily concentrated in the brain, wherein Sept3 is a substrate for PKG-I (cGMP-dependent protein kinase-I) in nerve terminals. There are two motifs for potential PKG-I phosphorylation in Sept3, Thr-55 and Ser-91, but phosphoamino acid analysis revealed that the primary site is a serine. Derivatization of phosphoserine to S-propylcysteine followed by N-terminal sequence analysis revealed Ser-91 as a major phosphorylation site. Tandem MS revealed a single phosphorylation site at Ser-91. Substitution of Ser-91 with Ala in a synthetic peptide abolished phosphorylation. Mutation of Ser-91 to Ala in recombinant Sept3 also abolished PKG phosphorylation, confirming that Ser-91 is the major site in vitro. Antibodies raised against a peptide containing phospho-Ser-91 detected phospho-Sept3 only in the cytosol of nerve terminals, whereas Sept3 was located in a peripheral membrane extract. Therefore Sept3 is phosphorylated on Ser-91 in nerve terminals and its phosphorylation may contribute to the regulation of its subcellular localization in neurons.