Lture and TransfectionsHEK-293T cells were maintained in DMEM supplemented with 10 FBS, 1 mM sodium pyruvate, and 1 mM penicillin/ streptomycin at 37uC in 5 CO2. HEK-293T cells were transiently transfected with full-length MERTK and kinase-dead R844C-MERTK using FuGENE as recommended (Roche). Rat RPE-J cells were maintained in Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 4 fetal bovine serum (FBS), and 1 mM non-essential amino acids at 33uC in 5 CO2. Rat Grb2 siRNAs were obtained as a Smartpool (Thermo Scientific) Dimethylenastron web containing mixtures of four different duplexes to minimize silencing of unintended targets. ON-TARGET plus non-targeting siRNA (at the same concentration as the total pool of targeting siRNAs) served as a negative control. RPE-J cells (32,000 cells per well) were passaged into eight-well chamber slides, and 24 h later each well was transfected with 0.5 mg of the siRNAs plus 3.75 mL of DharmaFect 3 transfection reagent as recommended (Dharmacon). The cells were incubated 18325633 with the siRNAs for 48 h, the medium was changed, and 24 h later the cells were transfected a second time and incubated for an additional 24 h. Cell viability was assessed by trypan blue staining, and was equivalent in cultures treated with targeting and nontargeting siRNAs. Phagocytosis assays were performed 5 days after siRNA transfection.rMERTK Expression and PurificationTwo His-tagged expression constructs encoding the human MERTK cytoplasmic domain, amino acid residues 571 to 864 (6xHis-rMERTK571?64) [23] and 571 to 24272870 999 (6xHisrMERTK571?99), in the pET28a-LIC vector were amplified in bacterial cells as described above for rSH2-domains, with kanamycin replacing ampicillin in the cultures. Cells were pelleted and resuspended in lysis buffer containing 50 mM Tris-HCl, 500 mM NaCl, 5 glycerol, 1 mM b-mercaptoethanol, 2 mM imidazole, and 200 mM phenylmethylsulfonyl fluoride (PMSF) at pH 8, and lysed by French press. Ni2+-NTA resin was incubated with cleared supernatants with MC-LR biological activity shaking for 1 h at 4uC, washed with 10 volumes of 10 mM imidazole in lysis buffer, and eluted with 200 mM imidazole in lysis buffer. The eluate was concentrated to 1 mL, chromatographed on Sephacryl S-200 HR as described above, evaluated on SDS gels, pooled, and concentrated. Recombinant MERTK was autophosphorylated by incubating with 10 mM ATP, 10 mM MgCl2 in gel filtration buffer at room temperature for 3 h and was stored at 280uC.Phagocytosis AssaysRod OS were isolated from bovine eyes [50] and covalently labeled with AlexaFluor 555 [52]. RPE-J cells were cultured for 6 days in eight-well chamber slides, and then incubated with 10 OS per cell for 4 h at 33uC. Unbound OS were removed by washing the cells 3 times with PBS containing 0.2 mM CaCl2 and 1 mM MgCl2, and the cells were fixed in 4 paraformaldehyde. To distinguish total and bound OS, duplicate samples were incubated before fixation with 0.2 trypan blue to quench fluorescence [53], as shown in Figure S2B. Slides were mounted using Prolong GoldPhosphotyrosine Analysis by MALDI-MSPurified, phosphorylated 6xHis-rMERTK571?64 was digested by addition of porcine trypsin in 50 mM ammonium bicarbonate, 0.05 SDS, and incubated overnight at 37uC. The digested peptides were subjected to TiO2 selection to enrich for phosphorylated peptides and evaporated to dryness in a SpeedVac. The sample was dissolved in 5 mL 60 Acetonitrile and 0.1 Trifluoroacetic acid. 1 mL of sample was spotted on MALDIMERTK Interactions with SH2-.Lture and TransfectionsHEK-293T cells were maintained in DMEM supplemented with 10 FBS, 1 mM sodium pyruvate, and 1 mM penicillin/ streptomycin at 37uC in 5 CO2. HEK-293T cells were transiently transfected with full-length MERTK and kinase-dead R844C-MERTK using FuGENE as recommended (Roche). Rat RPE-J cells were maintained in Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 4 fetal bovine serum (FBS), and 1 mM non-essential amino acids at 33uC in 5 CO2. Rat Grb2 siRNAs were obtained as a Smartpool (Thermo Scientific) containing mixtures of four different duplexes to minimize silencing of unintended targets. ON-TARGET plus non-targeting siRNA (at the same concentration as the total pool of targeting siRNAs) served as a negative control. RPE-J cells (32,000 cells per well) were passaged into eight-well chamber slides, and 24 h later each well was transfected with 0.5 mg of the siRNAs plus 3.75 mL of DharmaFect 3 transfection reagent as recommended (Dharmacon). The cells were incubated 18325633 with the siRNAs for 48 h, the medium was changed, and 24 h later the cells were transfected a second time and incubated for an additional 24 h. Cell viability was assessed by trypan blue staining, and was equivalent in cultures treated with targeting and nontargeting siRNAs. Phagocytosis assays were performed 5 days after siRNA transfection.rMERTK Expression and PurificationTwo His-tagged expression constructs encoding the human MERTK cytoplasmic domain, amino acid residues 571 to 864 (6xHis-rMERTK571?64) [23] and 571 to 24272870 999 (6xHisrMERTK571?99), in the pET28a-LIC vector were amplified in bacterial cells as described above for rSH2-domains, with kanamycin replacing ampicillin in the cultures. Cells were pelleted and resuspended in lysis buffer containing 50 mM Tris-HCl, 500 mM NaCl, 5 glycerol, 1 mM b-mercaptoethanol, 2 mM imidazole, and 200 mM phenylmethylsulfonyl fluoride (PMSF) at pH 8, and lysed by French press. Ni2+-NTA resin was incubated with cleared supernatants with shaking for 1 h at 4uC, washed with 10 volumes of 10 mM imidazole in lysis buffer, and eluted with 200 mM imidazole in lysis buffer. The eluate was concentrated to 1 mL, chromatographed on Sephacryl S-200 HR as described above, evaluated on SDS gels, pooled, and concentrated. Recombinant MERTK was autophosphorylated by incubating with 10 mM ATP, 10 mM MgCl2 in gel filtration buffer at room temperature for 3 h and was stored at 280uC.Phagocytosis AssaysRod OS were isolated from bovine eyes [50] and covalently labeled with AlexaFluor 555 [52]. RPE-J cells were cultured for 6 days in eight-well chamber slides, and then incubated with 10 OS per cell for 4 h at 33uC. Unbound OS were removed by washing the cells 3 times with PBS containing 0.2 mM CaCl2 and 1 mM MgCl2, and the cells were fixed in 4 paraformaldehyde. To distinguish total and bound OS, duplicate samples were incubated before fixation with 0.2 trypan blue to quench fluorescence [53], as shown in Figure S2B. Slides were mounted using Prolong GoldPhosphotyrosine Analysis by MALDI-MSPurified, phosphorylated 6xHis-rMERTK571?64 was digested by addition of porcine trypsin in 50 mM ammonium bicarbonate, 0.05 SDS, and incubated overnight at 37uC. The digested peptides were subjected to TiO2 selection to enrich for phosphorylated peptides and evaporated to dryness in a SpeedVac. The sample was dissolved in 5 mL 60 Acetonitrile and 0.1 Trifluoroacetic acid. 1 mL of sample was spotted on MALDIMERTK Interactions with SH2-.