The monomerization of the Purple protein, a member of the GFP-family
Green fluorescent protein (GFP) has been used extensively since its discovery in the 1960s to report and visualize gene expression. For years it has been the only known naturally occurring fluorescent pigment that is encoded by a single gene, making it extremely useful in various fields of biology, because the expression of this gene directly leads to the appearance of the fluorescent green color. Recently, however, many more proteins with similar properties to GFP, and available in a variety of colors, have been isolated from the class of marine organisms called Anthozoa, which includes the corals. This increase in the availability of colored proteins in the GFP family in turn has expanded the number of available biotechnology applications. However, some of these newly discovered GFP-like proteins do not have wild-type forms that readily allow for the creation of fusion proteins, particularly because of oligomerization. It is widely accepted that almost all members of the GFP-family form dimers or tetramers in their functional forms. This study investigates a GFP-like protein, Purple, isolated from two species, Galaxea fascicularis and Montipora efflorescens. Purple protein forms oligomers when expressed, which would then interfere with the normal expression of a protein to be tagged in gene fusion experiments. We selectively mutated 3 amino acids, which we believed were responsible for oligomerization in Purple. These 3 residues were chosen based on sequence similarities to a very similar protein, a mutant form of the Rtms5 chromoprotein from Montipora efflorescens. While we had hoped that the resulting triple-mutant Purple protein would form monomers in vivo while retaining its purple coloration, this turned out to be incorrect. The resulting mutants had lost their ability to turn purple. However, we also determined that we had successfully changed the oligomerization state of Purple by examining the relative molecular mass of one our mutant proteins, which turned out to be half the size of the original purple protein. It is possible that by adding addition mutations in the future, the original spectral properties could be recovered. If successful, this would further expand the utility of the GFP family.