not as such
they methylate or demethylate some residues (slips my mind which) on an internal protein complex in response to the presence or absense of "stuff" outside the cell - the complex acts through the cell membrane - and the level of methylation allows the cell a brief "memory", effectively, of the concentration of "stuff" outside the cell, so the cell can sense concentration gradients in "stuff" and know if it is moving itself towards or away from the "stuff" - which helps for foods and toxins.

/biochemistry of "stuff" and "moving" by TMB, aged 35 1/3.
( the mighty badger Aphrodite, on a bar stool, by your side, Wed 12 Jan 2011, 14:44, Reply)