The structure of KcsA is that of an inverted cone, with a central pore running down the center made up of two transmembrane helices (the outer-helix M1 and the inner-helix M2), which span the lipid bilayer.
The channel itself is a tetramer composed of four identical, single-domain subunits (each with two α-helices) arranged so that one M2 helix faces the central pore, while the other M1 helix faces the lipid membrane.
The inner helices are tilted by about 25° in relation to the lipid membrane and are slightly kinked, opening up to face the outside of the cell like a flower.
These two TM helices are linked by a reentrant loop, dispersed symmetrically around a common axis corresponding to the central pore.
The pore region spans approximately 30 amino acid residues and can be divided into three parts: a selectivity filter near the extracellular side,  a dilated water-filled cavity at the center, and a closed gate near the cytoplasmic side formed by four packed M2 helices.
 This architecture is found to be highly conserved in the potassium channel family in both eukaryotes and prokaryotes.

The overall length of the pore is 45 Å, and its diameter varies considerably within the distinct regions of the inner tunnel.
Travelling from the intracellular region outwards (bottom to top in the picture) the pore begins with a gate region formed by M2 helices at 18 Å in diameter, and then opens into a wide cavity (∼10 Å across) near the middle of the membrane.
In these regions, K⁺ ions are in contact with surrounding water molecules but when they enter the channel from the selectivity filter at the top, the cavity is so narrow that K⁺ ions must shed any hydrating waters in order to enter the cell.
In regards to the amino acid composition of the pore-lining residues within KcsA, the side chains lining the internal pore and cavity are predominantly hydrophobic, but within the selectivity filter polar amino acids are present that contact the dehydrated K⁺ ions.
The structure of KcsA is that of an inverted cone, with a central pore running down the center made up of two transmembrane helices (the outer-helix M1 and the inner-helix M2), which span the lipid bilayer. The channel itself is a tetramer composed of four identical, single-domain subunits (each with two α-helices) arranged so that one M2 helix faces the central pore, while the other M1 helix faces the lipid membrane. The inner helices are tilted by about 25° in relation to the lipid membrane and are slightly kinked, opening up to face the outside of the cell like a flower. These two TM helices are linked by a reentrant loop, dispersed symmetrically around a common axis corresponding to the central pore. The pore region spans approximately 30 amino acid residues and can be divided into three parts: a selectivity filter near the extracellular side,  a dilated water-filled cavity at the center, and a closed gate near the cytoplasmic side formed by four packed M2 helices.  This architecture is found to be highly conserved in the potassium channel family in both eukaryotes and prokaryotes.
The overall length of the pore is 45 Å, and its diameter varies considerably within the distinct regions of the inner tunnel. Travelling from the intracellular region outwards (bottom to top in the picture) the pore begins with a gate region formed by M2 helices at 18 Å in diameter, and then opens into a wide cavity (∼10 Å across) near the middle of the membrane. In these regions, K⁺ ions are in contact with surrounding water molecules but when they enter the channel from the selectivity filter at the top, the cavity is so narrow that K⁺ ions must shed any hydrating waters in order to enter the cell. In regards to the amino acid composition of the pore-lining residues within KcsA, the side chains lining the internal pore and cavity are predominantly hydrophobic, but within the selectivity filter polar amino acids are present that contact the dehydrated K⁺ ions.
KcsA potassium channel