Ions cannot easily move in and out of cells, so protein channels such as voltage-gated potassium channels and calcium-permeable anion channels are responsible for allowing potassium and calcium, respectively, to flow through the cell membrane, making cells permeable to these ions. The action potential causes potassium ions to flow out from the vacuoles of cells in the various pulvini. Differences in turgidity in different regions of the leaf and stem results in the closing of the leaflets and the collapse of the leaf petiole. Other important proteins include H+-ATPases, aquaporins, and actin, which all aid in the redistribution of ions in the pulvini, especially during a seismonastic response. H+-ATPases and aquaporins aid in the direct movement of water molecules, while actin's role has a more biochemical explanation. Actin is composed of many phosphorylated tyrosine (an amino acid) molecules, and manipulation of how phosphorylated the tyrosine molecules are directly correlates to how much the M. pudica leaves droop.