It was found that bacterial transglutaminase (TGase) facilitates selective
cross-linking of bacteriorhodopsin (BR) in purple membrane (PM) form under
mild conditions. Fluorescent probes were used to detect that the membrane p
rotein BR may act as a glutamine donor as well as a lysine donor for TGase.
The binding sites were determined to be Gln-3 as the reactive glutamine, a
nd Lys-129 is the corresponding lysine residue. Upon incubation of PM with
TGase, cross-linking of PM patches can be achieved without an additional sp
acer molecule. To our knowledge, this is the first time that an intermembra
ne cross-linking of membrane bound proteins is reported. Furthermore, this
finding may provide the ability to achieve covalent linkage of complete pur
ple membrane patches to synthetic polymers.