 Both p90 and p70 Rsk phosphorylate ribosomal protein s6, part of the translational machinery, but several other substrates have been identified, including other ribosomal proteins.Cytosolic substrates of p90rsk include protein phosphatase 1; glycogen synthase kinase 3 (GSK3); L1 CAM, a neural cell adhesion molecule; Son of Sevenless, the Ras exchange factor; and Myt1, an inhibitor of cdc2.RSK phosphorylation of SOS1 (Son of Sevenless) at Serines 1134 and 1161 creates 14-3-3 docking site.