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Fig. 4. LSP1-associated MEK1 is activated by anti-IgM. Anti-MEK1 (A) or anti-LSP1 (B) immunoprecipitates were analyzed for kinase activity using a peptide containing the MEK phosphorylation site of human ERK1 as substrate. Similarly, anti-ERK2 (C) or anti-LSP1 (D) immunoprecipitates were analyzed for kinase activity using an EGF-derived peptide. Precipitates were prepared from CHAPS-soluble lysates from unstimulated cells or from cells stimulated for 5, 10 or 20 minutes with 50 µg/ml anti-IgM ({square}, W10; {blacksquare}, TW10.1; {blacktriangledown}, W10 cells pretreated for 20 minutes with 10 µM U0126). Results are expressed as the average fold increase over unstimulated cells±s.e.m. of three experiments. Anti-MEK1 (E) and anti-LSP1 (F) immunoprecipitates were assayed for MEK1 activity using recombinant His-ERK2 as substrate. Anti-ERK2 (G) and anti-LSP1 (H) were assayed for ERK2 activity using myelin basic protein (MBP) as a substrate.