Protein cyclization enhanced thermostability and exopeptidase-resistance of green fluorescent protein.

A mutant of green fluorescent protein (GFPmut3*) from the jellyfish Aequorea victoria was cyclized in vitro and in vivo by the use of a naturally split intein from the dnaE gene of Synechocystis species PCC6803 (Ssp). Cyclization of GFPmut3* was confirmed by amino acid sequencing and resulted in an increased electrophoretic mobility compared with the linear GFPmut3*. The circular GFPmut3* was 5 degrees C more thermostable than the linear form and significantly more resistant to proteolysis of exopeptidase. The circular GFPmut3* also displayed increased relative fluorescence intensity. In addition, chemical stability of GFPmut3* against GdnHCl revealed more stability of the circular form compared with the linear form.