The serine/threonine protein phosphatases are targeted to specific subcellular locations and substrates in part via interactions with a wide variety of regulatory proteins. Understanding these interactions is thus critical to understanding phosphatase function. Using an iterative affinity purification/mass spectrometry approach, we generated a high density interaction map surrounding the protein phosphatase 2A catalytic subunit. This approach recapitulated the assembly of the PP2A catalytic subunit into many different trimeric complexes but also revealed several new protein-protein interactions. Here we define a novel large multiprotein assembly, referred to as the striatin-interacting phosphatase and kinase (STRIPAK) complex. STRIPAK contains the PP2A catalytic (PP2Ac) and scaffolding (PP2A A) subunits, the striatins (PP2A regulatory Bٟ subunits), the striatin-associated protein Mob3, the novel proteins STRIP1 and STRIP2 (formerly FAM40A and FAM40B), the cerebral cavernous malformation 3 (CCM3) protein, and members of the germinal center kinase III family of Ste20 kinases. Although the function of the CCM3 protein is unknown, the CCM3 gene is mutated in familial cerebral cavernous malformations, a condition associated with seizures and strokes.

Metazoan genomes encode hundreds of RNA-binding proteins (RBPs) but RNA-binding preferences for relatively few RBPs have been well defined. Current techniques for determining RNA targets, including in vitro selection and RNA co-immunoprecipitation, require significant time and labor investment. Here we introduce RNAcompete, a method for the systematic analysis of RNA binding specificities that uses a single binding reaction to determine the relative preferences of RBPs for short RNAs that contain a complete range of k-mers in structured and unstructured RNA contexts. We tested RNAcompete by analyzing nine diverse RBPs (HuR, Vts1, FUSIP1, PTB, U1A, SF2/ASF, SLM2, RBM4 and YB1). RNAcompete identified expected and previously unknown RNA binding preferences. Using in vitro and in vivo binding data, we demonstrate that preferences for individual 7-mers identified by RNAcompete are a more accurate representation of binding activity than are conventional motif models. We anticipate that RNAcompete will be a valuable tool for the study of RNA-protein interactions.

Fibroblasts differentiate into the highly synthetic and contractile myofibroblast phenotype when exposed to substrates with an elastic modulus corresponding to pathologically stiff fibrotic tissue. Cellular responses to changes in substrate stiffness are typically analyzed after hours or days, which does not enable the monitoring of myofibroblast persistence, a hallmark of fibrosis. To determine long-lasting effects on the fibrotic behavior of lung fibroblasts, we followed a novel approach of explanting and repeatedly passaging fibroblasts on silicone substrates with stiffness representing various states of lung health. Fibrotic activity was determined by assaying for myofibroblast proliferation, cell contractility, expression of α-smooth muscle actin, extracellular matrix and active TGFβ1. As predicted, myofibroblast activity was low on healthy soft substrates and increased with increasing substrate stiffness. However, explanting and mechanically priming lung fibroblasts for 3 weeks on pathologically stiff substrates resulted in sustained myofibroblast activity even after the cells were returned to healthy soft cultures for 2 weeks. Such primed cells retained higher fibrotic activity than cells that had been exclusively cultured on soft substrates, and were not statistically different from cells continuously passaged on stiff surfaces. Inversely, priming lung fibroblasts for 3 weeks on soft substrates partially protected from myofibroblast activation after the shift to stiff substrates. Hence, mechano-sensed information relating to physical conditions of the local cellular environment could permanently induce fibrotic behavior of lung fibroblasts. This priming effect has important implications for the progression and persistence of aggressive fibrotic diseases such as idiopathic pulmonary fibrosis.

BackgroundMembranous nephropathy (MN) results from deposition of antigen-antibody complexes along the glomerular basement membrane (GBM). PLA2R, THSD7A, NELL1, and SEMA3B account for 80%–90% of target antigens in MN.MethodsWe performed laser microdissection and mass spectrometry (MS/MS) in kidney biopsies from 135 individuals with PLA2R-negative MN, and used immunohistochemistry/immunofluorescence and confocal microscopy to confirm the MS/MS finding, detect additional cases, and localize the novel protein. We also performed MS/MS and immunohistochemistry on 116 controls and used immunofluorescence microscopy to screen biopsy samples from two validation cohorts. Western blot and elution studies were performed to detect antibodies in serum and biopsy tissue.ResultsMS/MS studies detected a unique protein, protocadherin 7 (PCDH7), in glomeruli of ten (5.7%) PLA2R-negative MN cases, which also were negative for PLA2R, THSD7A, EXT1/EXT2, NELL1, and SEMA3B. Spectral counts ranged from six to 24 (average 13.2 [SD 6.6]). MS/MS did not detect PCDH7 in controls (which included 28 PLA2R-positive cases). In all ten PCDH7-positive cases, immunohistochemistry showed bright granular staining along the GBM, which was absent in the remaining cases of PLA2R-negative MN and control cases. Four of 69 (5.8%) cases in the validation cohorts (all of which were negative for PLA2R, THSD7A, EXT1, NELL1, and SEMA3B) were PCDH7-positive MN. Kidney biopsy showed minimal complement deposition in 12 of the 14 PCDH7-associated cases. Confocal microscopy showed colocalization of PCDH7 and IgG along the GBM. Western blot analysis using sera from six patients showed antibodies to nonreduced PCDH7. Elution of IgG from frozen tissue of PCDH7-associated MN showed reactivity against PCDH7.ConclusionsMN associated with the protocadherin PCDH7 appears to be a distinct, previously unidentified type of MN.

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