Designed by: Liu Xu   Group: iGEM19_ECUST_China   (2019-10-16)

HlyA-tag (Flexible linker)

The alpha-hemolysin system is one of the best-studied type 1 secretion systems (T1SS) of E. coli. In T1SS the secretion occurs in a single step directly from the cytosol to the extracellular medium. The secretory machinery of the alpha-hemolysin system consists of three proteins: HlyB, an ATP binding cassette; HlyD, a membrane fusion protein; and TolC, an outer membrane protein.The natural substrate HlyA can be secreted because it contains a C-terminal signal and it has been shown that proteins with C-terminally fused HlyA signal sequence can also be recognized by the HlyB-HlyD-TolC translocator.

Usage and Biology

iGEM13_tecMonterrey team has devised a composite part BBa_K1166002, which combined the hlyA-tag, arabinose-inducible hlyB and hlyD as a whole. Yet the characterization results from iGEM13_tecMonterrey team has shown that,

1).the hlyA-tag could affect the conformation of GFP(the target protein in this case), thus leading to the fluorescence quenching of GFP. This problem implied that, the hlyA-tag might influence the conformation and even function of the target protein when expressed as fusion protein;

2). the hemolysin secretion system is sufficient only to small range of protein.

Given these defects existing in part BBa_K1166002, we have improved this part by amending 3 different types of alternative linker—flexible linker, rigid linker, and OmpT-cut linker—on the basis of original double-GS linker.


—The flexible linker BBa_K3093010 consists of three GS repeats, aiming to join target gene with hlyA-tag while providing larger degree of movement or interaction.

Experiment results

Figure1:clones of DH5α transformants (pIN2 + mRFP + linker + hlyA + araC + pBAD + hlyB + hlyD) of 4 types of linker on LB-kan plates and in liquid M9 media.

All the colonies proved to be positive by colony PCR and first generation sequencing could be easily spotted red on plates and liquid media, directly indicating that the all the linker+hlyA tags were not interfering the emission of fluorescence of mRFP. Compared with using GFP as the secreted target gene, mRFP is apparently more suitable for the fusion of hlyA to testify the secretion efficiency of hemolysin system.

Figure-2:liquid LB media containing positive colonies with(Y) or without(N) arabinose induction after 16 hours. Left three media are pIN2+mRFP, and pIN2(empty vector) as negative control.

Positive transformants with different linker were cultured and induced by arabinose. It’s conspicuous that the arabinose induced E.coli grew much slower, implying expressing HlyB and HlyD was an addictive burden for the engineered cell.

We collected the same weight of E.coli by restricting the value(OD multiple volume) equal to 16.32. And then separate the media and cell by centrifugation. The supernate were concentrated 100 fold and the pellet were resuspended with PBS. Supernate and pellet were both pretreated to prepare protein sample and run SDS-PAGE.

Figure-3:SDS-PAGE of supernatant and pellet of types of linker-transformants.

The SDS-PAGE showed that, the fusion protein—mRFP-hlyA(in the red frame, about 32.6kD)—were all expressed inside the cell and partially successfully secreted to the media. And in the OmpT-cleavable linker case, the mRFP and hlyA-tag were also successfully cleaved, since the cleaved mRFP(in the rgreen frame, about 26.0kD) clearly surpassed the fusion protein on the gel. Although the mRFP-hlyA fusion protein and mRFP were both slightly smaller than expected size, we highly suspected that was because, the pI of the mRFP-hlyA and mRFP were estimated to be 5.12 and 5.07 using an online tool(ExPASy), both lower than pH6.8, which might potentially affect the mobility ratio of these protein in the SDS-PAGE buffer and appeared on a lower molecular weight band on the gel.


So we’ve demonstrated the our improved part, BBa_K3093010, HlyA-tag+Secretion system (OmpT-cleavable linker BBa_K3093011) to be functional, and the Rigid linker BBa_K3093012 and the Flexible linker BBa_K3093013 were also proved to be not hindering the secretion of fusion protein while offering several alternative options for perspective users who want to express their protein in an secretory form. We are also looking forward to more application of our secretory system package! See more info at our improve-ECUST homepage.


[1]Chen X, Zaro JL, Shen WC. Fusion protein linkers: property, design and functionality. Adv Drug Deliv Rev. 2013 Oct;65(10):1357-69. doi: 10.1016/j.addr.2012.09.039. Epub 2012 Sep 29. Review. PubMed PMID: 23026637; PubMed Central PMCID: PMC3726540.

Sequence and Features

Sequence and Features

Assembly Compatibility:
  • 10
  • 12
  • 21
  • 23
  • 25
  • 1000