[1] With that, research has begun to prove that proteins function in supramolecular complexes compared to isolated entities.

[2] Scientists Miquel Vila-Perello´, Matthew R. Pratt, Frej Tulin, and Tom W. Muir wanted to create an efficient synthesis as the original had required an enzymatic solution and had a low yield.

[3] It was used then (with a protection group Fmoc on the amine) which that product underwent more synthetic steps to study if an amino-acid cross linker and a post-translational modification (PTM) could be introduced to the same protein site specifically to capture a covalent interaction of the amino-acid is dependent on the PTM.

[3] PTM's regulate protein-protein interactions that have characteristics that are transient and substoichiometric; making these difficult to detect by standard methods.

[3] So, in order to see if it would work, the MH2 domain of Smad2 was used because this signaling protein is known to form stable homo-trimers once they come into contact with receptor-phosphorylated serine residues.

The product was studied with the cross-linker (photo-Met) against a control protein: HA-MH2-CSpSMpS (this lacks photo-methionine, 2) using SDS-PAGE and western blotting using anti-HA antibody.

[3] As mentioned before, scientists Monika Suchanek, Anna Radzikowski, and Christoph Thiele wanted to study protein-protein interaction in their natural environment.

This method of combined photo-initiated cross-linking from the protein nanoprobe in tandem with MS could be useful to characterize not only homodimer formation but also oligomers and in theory; heteromers (such as the composition of the protein-protein mixture and its functionality).

[4] Cytochrome b5 was synthesized with photo-methionine to map the protein-protein interactions while also identifying its structure to study the mammalian mixed function oxidase system (also known as the MFO).

[5] A typical cross-linking method can only work in solvent exposed regions, proving once again that photo-methionine is useful to map these protein-protein interactions with the protein in their native environment.

[5] Laminin's are non-collagenous proteins found in basement membranes and form networks through non-covalent self-interactions.

Nidogens (also known as entactins) are sulfated monomeric glycoproteins that are ubiquitously present in basement membranes of higher organisms.

Even though it did not show any protein-protein interactions, it could be used to detect inhibitor-induced protein conformational changes in the cell membranes on top of determining oligomeric structures.

[7] Photo-methionine can be used to label recombinant proteins in Escherichia coli cells; though methionine in general is a rare amino acid so that means it could only give limited structural data.

[8] Nevertheless, photo-methionine was incorporated into Ca2+ regulating protein calmodulin (CaM that was 17-kDa) that has nine methionine's and studied via mass spectrometry (MS).