Shedding of LAR and PTPσ E subunits in A431 cells. A431 cells were starved for 2 d, washed once with starvation medium, and treated with or without A23187 (10 −5 M, 1 h) or TPA (1 μM, 40 min). Cell lysates ( TRITON ) were either immunoprecipitated with antiserum specific for LAR and PTPσ (IP:320, 0.9 mg protein) or were bound to WGA-sepharose beads (WGA, 0.6 mg protein). Proteins of the tissue culture supernatant of these cells ( MEDIUM , corresponding to 1.5 mg protein of cell lysate) were bound to WGA-sepharose beads ( WGA ). Immunoprecipitates and WGA-bound proteins were separated by 8% SDS-PAGE, transferred to nitrocellulose, and analyzed by immunoblotting of the membrane with antiserum specific for the COOH terminus of LAR and PTPσ ( 320 ), the NH 2 terminus of PTPσ ( 322 ), or the COOH terminus of the LAR E subunit ( αLAR EC ). Arrows at the right indicate the position of the LAR or PTPσ subunits.