"Alone we can do so little; together we can do so much." - Helen Keller

Two-Hybrid System

Figure 1: Plate streaking of transformed BTH101 on LB/X-gal plates, containing pSB1C3-T18 + pSB1K3-T25, pSB1C3-T18-Zip + pSB1K3-T25, pSB1C3-T18-Zip + pSB1K3-T25 and pSB1C3-T18-Zip + pSB1K3-T25-Zip.

In order to make a great screening for peptide aptamers you need a well-functioning screening system. Our screening system is the bacterial two-hybrid system, which is based on the reconstitution of the adenylate cyclase and enables detection of protein-protein interaction. Here we validate the function of our bacterial two-hybrid system.

To validate whether our T18 and T25 domain constructs in fact can be used to study protein-protein interactions, we made a control experiment, where the leucine zipper (Zip) region from the GCN4 yeast (Saccharomyces cerevisiae) protein was fused to the T18 and T25 domains (T18-Zip+T25-Zip). Leucine zippers are known to interact by forming homodimers and if the system indeed works, their interaction will lead to functional complementation between the T18 and T25 domains. This leads to the synthesis of cAMP, and thus by using a cAMP-induced reporter system, one can observe whether or not there is an interaction.

When working with the bacterial two-hybrid system, it is necessary to use a strain with a non-functioning or absent adenylate cyclase, cyaA. For this purpose we constructed the Escherichia coli K12-strain, MG1655 ΔcyaA. We also used the cyaA-deficient E. coli K12-strain BTH101 (MC1061-derived). This was for the purpose to exploit the lacZ-derived reporter system encoded in this strains chromosome. The lacZ gene encodes a β-Galactosidase which is positively controlled by cAMP. Our goal was to use the RFP reporter system in the MG1655 ΔcyaA-strain. Due to difficulties with RFP reporter system, this control experiment was carried out in the BTH101-strain.

As expected, the results only displayed complementation between T18 and T25 when the leucine zipper was fused to both of the domains. These results infers that our constructs function as expected, and that they can be used to detect protein-protein interactions. This indicates that the system can be used to screen for peptide aptamers. If we wanted to use the RFP reporter system instead, the same experiment could be conducted and red colonies would be observed instead of blue. The transformations should, however, be plated on LB plates without X-gal.

We wanted to carry out a screening for peptide aptamers, but due to delayed deliverance of our nucleotide library and problems inserting it into our scaffold, we did not manage to accomplish this in the given time frame.

Figure 2: Fluorescence microscopy images: (A) pSB1C3-T25-GFP, (B) pSB1C3-T18-GFP and (C) pSB1C3-T18 transformed into Top10 (E. coli K12-strain).

Green Fluorescent Protein was fused to the two components of the two-hybrid system, T18 and T25. This allowed us to detect expression of the two components that are both controlled by the lac promoter. Additionally, the presence of green fluorescence will verify that proteins can be fused to T18 and T25, and still fold into the correct structure. The two constructs pSB1C3-T18-GFP and pSB1C3-T25-GFP were transformed into the E. coli K12-strain Top10. The following fluorescence microscopy images confirm the presence of green fluorescence. This means that both T18 and T25 are expressed from the lac promoter, and that proteins fused to these two constructs can fold into their native structure.

The two constructs, pSB1C3-T18-GFP and pSB1C3-T25-GFP, were transformed into the competent E. coli K12-strain, Top10. Fluorescence microscopy imaging (fig. 2) confirms the presence of green fluorescence, thus both T18 and T25 are expressed by the lac promoter, and that GFP fused to the two constructs can fold into its correct structure. This indicates that proteins can be fused to T18 and T25, without affecting their 3D-structure.