In Vivo N-Terminal Functionalization of Protein Figure 2. Comparison of functional productivity of GFPnt and its variants. The data show the whole cell fluorescences of GFPnt (containing all the 5 internal Met), GFPnt-r2M (containing M153T and M233K mutations), GFPnt-r3M (containing M153T, M233K and M218A mutations), GFPnt-r4M (M78I, M88L, M153T and M233K mutations), GFPnt-r5M (M78I, M88L, M153T, M218A and M233K mutation) and GFPhs-r5M (GFPnt-r5M containing mutations for folding enhancement). The relative fluorescence (in arbitrary units) is the fluorescence of whole cells compared with the fluorescence of cells expressing GFPnt. doi:10.1371/journal.pone.0046741.g002 times lower than that of GFPnt-r2M (Figure 2). SDS-PAGE analysis confirmed that the reduced fluorescence of GFPnt-r3M was caused by a misfolding of the protein (Figure S1B), which highlights the importance of the M218 residue in the folding of GFP. Similarly, the other two internal Met positions (M78 and M88) in GFPnt-r2M were randomized at the same time with hydrophobic amino acids (Leu, Ile, Phe, Val, and Ala). A GFPnt- r2M variant having the M78I and M88L mutations, designated as GFPnt-r4M, showed the highest fluorescence; cells expressing GFPnt-r4M exhibited around 3-fold lower fluorescence than those expressing GFPnt-r2M (Figure 2). This result suggests that the M78 and M88 residues in the hydrophobic core are also important in GFP folding. All the three mutations, M78I, M88L, and M218A, were introduced into GFPnt-r2M, which resulted in a complete internal Met-free GFP sequence, GFPnt-r5M. However, the whole cell fluorescence of GFPnt-r5M was approximately 7 times lower than that of GFPnt-r2M (Figure 2), and GFPnt-r5M was mostly expressed as an insoluble form (Figure S1C). This confirms that the three Met residues in the hydrophobic core are very important in the formation of active GFP structure. Although it was not successful to generate an internal Met-free protein with preserved initial activity, these results suggest that the semi-rational approach based on similar physicochemical amino acids can be a handy tool for engineering a protein devoid of internal Met. Both the three mutations M78L, M88F, and M218A in GFPrm_AM, and the mutations found in this study (M78I, M88L, and M218A) did not result in an active internal Met-free GFP variant. One thing that needs to be noted is that the starting GFP sequence to generate GFPrm_AM is a GFP variant (L024_3- 3) that exhibited higher expression, better refolding behavior and higher stability than normal GFP [27], and thus we suspected that the properties of template GFP sequence could be an important factor for succeeding in generating an internal Met-free GFP variant. Since L024_3-3 was engineered to make GFP fluorescent with 5,5,5-trifluoroleucine, we turned to another GFP variant, superfolder GFP [19], which also showed improved folding properties and much more resistance to mutations than a wild type GFP. We introduced the mutations of superfolder GFP (S30R, Y39N, F64L, F99S, N105T, Y145F, M153T, V163A, I171V, and A206V) into GFPnt-r5M. It was also reported that N149K [28] and S208L [29] affected the folding efficiency of GFP positively, although their effects were not significant. The two mutations (N149K and S208L) were additionally introduced, and the resulting variant was named GFPhs-r5M. As shown in the Figure 2, the whole cell fluorescence of GFPhs-r5M was much higher than that of GFPnt-r5M, and approximately 2.5 times higher than GFPnt. SDS-PAGE analysis of the expressed protein confirmed that the soluble expression level of the GFPhs-r5M protein was improved significantly compared to that of GFPnt-r5M and higher than that of GFPnt (Figure S1D), suggesting that the introduced mutations improved the folding efficiency of GFPnt- r5M remarkably. Table S2 shows the protein sequence of the soluble and active internal Met-free variant, i.e. GFPhs-r5M. N-terminal Functionalization of the Internal Met-free GFP The GFPhs-r5M variant obtained from the above study is expressed as a functional form, and contains a Met residue only in its N-terminus, which suggests that the expression of the gene for GFPhs-r5M using the Met residue substitution method may enable the production of N-terminal functionalized GFP in vivo. To demonstrate this, the gene for GFPhs-r5M was expressed in the Met auxotrophic E. coli with the addition of Met surrogates, Hpg or Aha, according to the previously reported procedures [14]. Hpg and Aha are unnatural amino acids containing alkyne and azide groups respectively, which are illustrated in Figure S2. The soluble expression of GFPhs-r5M with Hpg or Aha was confirmed by SDS-PAGE (Figure 3A) and the corresponding active fluorescent proteins were produced despite an approximately 20% decrease in whole cell fluorescence compared to GFPhs-r5M with Met (Figure 3B). The proteins produced were purified and analyzed PLOS ONE | www.plosone.org 4 October 2012 | Volume 7 | Issue 10 | e46741