Brk mediates Met receptor signaling to ERK5. HaCaT (a) and MDA-MB-231 (b) cells were transiently transfected with a nontargeting siRNA (control) or Brk siRNA and serum-starved for 24 hr. Cells were then treated with vehicle (water) or 50 ng/ml HGF for 15 to 60 min. Whole cell lysates were harvested and subjected to Western blotting with phospho- and total-specific antibodies to AKT, ERK5, ERK1/2, p-38, and total Brk. Cells were stimulated with 20 ng/ml EGF for 15 min as a positive control for kinase activation.