Lysine (LYS or K) is a hydrophilic amino acid. Its shape in Foldit is unique. It's longer than methionine.
Lysine has a tendency to form part of a helix, as part of the "MALEK" group of amino acids.
In the Foldit Lua interface, the functions structure.GetAminoAcid and structure.SetAminoAcid use lowercase "k" for lysine.
For more, see Lysine on wikipedia.