Figure 1.

MEKK1 selectively activates JNK and p38. Cos-1 cells were transiently transfected with expression vectors for HA-tagged JNK1 (A,D), p38α (B,E), or ERK2 (C,F), together with increasing amounts (in ng) of either ΔMEKK1 (A,B,C) or FL-MEKK1 (D,E,F) vectors. After 48 hr, 10-μg samples of transfected cell lysates were used to determine MAPK expression by immunoblotting with anti-HA (bottom panels). Fifty-microgram samples were subjected to immunecomplex kinase assays using anti-HA and appropriate recombinant proteins as substrates (top panels). Fold stimulations of MAPK activities were calculated after phosphoimaging and normalization for MAPK expression levels. As a reference, JNK1 was stimulated by UV irradiation (40 J · m⁻²), p38α by UV irradiation or anisomycin (1 μg/ml), and ERK2 by treatment with phorbol ester (TPA, 10 ng/ml).