Protein layers and their use in X-ray crystallography

Protein layers 1 repeating regularly in two dimensions comprise protein protomers 2 which each comprise at least two monomers 5, 6 genetically fused together. The monomers 5, 6 are monomers of respective oligomer assemblies 3, 4 into which the monomers are assembled to assembly of the protein layer. The first oligomer assembly 3 belongs to a dihedral point group of order O, where O equals 3, 4 or 6 and has a set of O rotational symmetry axes of order 2. The second oligomer assembly 4 has a rotational symmetry axis of order 2. Due to the symmetry of the oligomer assemblies 3, 4, the rotational symmetry axes of each second oligomer assembly 4 is aligned with one of said set of O rotational symmetry axes of a first oligomer assembly 3 with 2 protomers being arranged symmetrically therearound. Thus, an 2-fold fusion between the oligomer assemblies 3, 4 is produced and the arrangements of the rotational symmetry axes of the oligomer assemblies 3, 4 cause the protein layer to repeat regularly. The protein layer has many uses, for example to support molecular entities for biosensing, x-ray crystallography or electron microscopy.