The 70kDa ribosomal protein S6 kinase 1 (S6K1) plays important roles in the 
regulation of protein synthesis, cell growth and metabolism. S6K1 is activated 
by the phosphorylation of multiple serine and threonine residues in response to 
stimulation by a variety of growth factors and cytokines. In addition to 
phosphorylation, we have recently shown that S6K1 is also targeted by lysine 
acetylation. Here, using tandem mass spectrometry we have mapped acetylation of 
S6K1 to lysine 516, a site close to the C-terminus of the kinase that is highly 
conserved amongst vertebrate S6K1 orthologues. Using acetyl-specific K516 
antibodies, we show that acetylation of endogenous S6K1 at this site is potently 
induced upon growth factor stimulation. Although S6K1 acetylation and 
phosphorylation are both induced by growth factor stimulation, these events 
appear to be functionally independent. Indeed, experiments using inhibitors of 
S6K1 activation and exposure of cells to various stresses indicate that S6K1 
acetylation can occur in the absence of phosphorylation and vice versa. We 
propose that K516 acetylation may serve to modulate important kinase-independent 
functions of S6K1 in response to growth factor signalling.