(a) Constitutively active RagA and RagB stimulate S6K phosphorylation. 200 ng of each mammalian RagA, RagB, RagC or RagD construct was co-transfected with HA-S6K (20 ng) into HEK293 cells. Their corresponding dominant-negative mutants (RagAT21N, RagBT54N, RagCS75N, RagDS76N) and constitutively active mutants (RagAQ66L, RagBQ99L, RagCQ120L, RagDQ121L) were also tested. Phosphorylation and protein levels were determined by immunoblotting with the appropriate antibodies, as indicated. 