| Assaying ERK 5 activity in immune complexes with one of these substrates , c Myc , we determined that the ERK 5 catalytic domain is activated by V 12 H Ras and to a lesser extent by phorbol ester but not by constitutively active mutants of Raf 1 . ^^^ Thus , ERK 5 is a target of a novel Ras effector pathway that may communicate with c Myc . . ^^^ |