ABSTRACT: Human keratinocytes, HaCaT cells were transfected with the dsRNA-analogue polyinosinic-polycytidylic acid (poly I:C) (Sigma-Aldrich) using Lipofectamine 2000 reagent (Invitrogen) for intracellular delivery. The cells were transfected with 7 ug/ml poly I:C for 1 h or left untreated. The cells were collected in PBS with a cell scraper and washed once with PBS before they were lysed with HEPES lysis buffer (50 mM HEPES, 150 mM NaCl, 1 mM EDTA, 1 % NP-40, pH 7.4) including protease and phosphatase inhibitor cocktails (Sigma-Aldrich). The cell lysates were centrifuged 2,264 x g for 15 min at 4°C, divided into smaller fractions and centrifuged again 11,686 x g for 15 min at 4°C . The supernatant was collected and the protein content was measured with Bio-Rad DC protein assay (Bio-Rad). For the samples, 8 mg of protein was used. The proteins were precipitated with 10 % TCA/acetone at -20°C overnight. Samples were centrifuged 10,733 x g for 20 min at 4°C and the supernatant was removed. The precipitation was washed once with acetone and dissolved in urea buffer (8 M urea, 400 mM NH4CO3, 20 mM DL-dithiotheitol, 1 mM EDTA, pH 8.5). The proteins were reduced, alkylated, and enzymatically digested in-solution with lysyl endopeptidase (Wako Chemicals) and trypsin (Promega). The peptides were fractionated by strong cation exchange chromatography. Phosphopeptide enrichment was performed with immobilized-metal affinity chromatography (IMAC) using PHOS-Select Iron Affinity Gel (Sigma Aldrich). The enriched phosphopeptides were analyzed by nanoLC-MS/MS using an Ultimate 3000 nano-LC (Dionex) coupled to a QSTAR Elite hybrid quadrupole TOF-MS (Applied Biosystems/MDS Sciex) with nano-ESI ionization.23,24 The LC-MS/MS data were submitted through the ProteinPilot 4.0 interface (Applied Biosystems/MDS Sciex) to an in-house Mascot database search engine version 4.0 (Matrix Science), and to the ProteinPilot algorithm Paragon. The data were searched against the human canonical sequences in the Swiss-Prot database (decoy version 08132012 for the Mascot searches and version 04012013 with 539,829 sequences for the Paragon searches). The Mascot search criteria were: fixed modifications: carbamidomethyl of cysteine, variable modifications: oxidation (M), phospho (ST), phospho (Y), acetylation (K), 1 missed cleavage allowed, enzyme: trypsin. The Paragon search criteria were: modifications: cystein alkylation with iodoacetamide, ID focus: biological modifications, phosphorylation emphasis, identification threshold: 95 % confidence (Unused ProtScore (Conf) > 1.3), Thorough search (ID), enzyme: trypsin.
Tuula Nyman 
