sentence: The protein binds to the N-terminus of Raf-1 , preventing its interaction with MEK , an event that is required for MEK activation [ 13,14,11 ] ; the association also impedes phosphorylation of Raf-1 at serine 338 ( S338 ) and tyrosine 341 ( Y341 ) [ 15 ] , mandatory events for Raf-1 activation [ 16,17 ] .