Difference between revisions of "Part:BBa K190028"

Revision as of 22:40, 21 October 2009

GlpF

[http://2009.igem.org/Team:Groningen/Project/Transport#GlpF GlpF] is an aquaglyceroporin channel that facilitates the transport of As(III). GlpF is an aquaglycerol porin of E.coli which facilitates not only glycerol import, but also arsenic (As) and antimone (Sb) import (Fu, DX, et al.2000, Meng, YL, et al.2004, Porquet, A, et al.2007, Rosen, BR, et al.2009). It has homologues in other organisms; Fps1p has shown to facilitate arsenic import in yeast and AQP9 is the mammalian homologue (Porquet, A, et al.2007, Rosen, BR, et al.2009). The GlpF aquaglycerol porin is a membrane protein with a symmetric arrangement of four independent GlpF channels. One monomer of this tetramer GlpF porin consists of six transmembrane and two half membrane-spanning α-helices that form a right-handed helical bundle around the channel. The channel has a diameter of ~15Å at the periplasmid end, which constricts towards a diameter of ~3.8Å at the beginning of a 28 Å long selective channel that ends at the cytoplasmic end (Fu, DX, et al.2000). The GlpF is a stereospecific channel that is thought to be more selective on molecular size than on chemical structure (Fu, DX, et al.2000, Heller, KB, et al.1980). It does allow transport of a variance of polyhydric alcohols, glycerol being one of them, and arsenic and antimone (Fu, DX, et al.2000, Meng, YL, et al.2004, Porquet, A, et al.2007,Rosen, BR, et al.2009, Heller, KB, et al.1980). Carbon sugars and ions are shown to be unable to be transported by GlpF (Heller, KB, et al.1980). At physiological pH arsenic and antimone are not present in their ionic state but rather as As(OH)3 and Sb(OH)3 (Rosen, BR, et al.2009). These elements show a charge distribution similar to glycerol and a smaller but comparable volume. The structural similarities are thought to be the reason for the possibility of these elements to be transported in the cell by GlpF (Porquet, A, et al.2007). If GlpF behaves as a nonsaturable transporter, a transport rate of 1umol of glycerol is transported per minute per mgr of cell protein (Heller, KB, et al.1980). The transport rate of GlpF for arsenic is estimated to be..

references:

• [http://2009.igem.org/Team:Groningen/Literature#Meng2004 Meng 2004]
• [http://2009.igem.org/Team:Groningen/Literature#Rosen2009 Rosen 2009]
• [http://2009.igem.org/Team:Groningen/Literature#Porquet,A,etal2007 Porquet, A, et al.2007]
• [http://2009.igem.orgTeam:Groningen/Literature#Fu,DX,etal2000 Fu, DX, et al.2000]
• [http://2009.igem.orgTeam:Groningen/Literature#Heller,KB,etal1980 Heller, KB, et al.1980]

Sequence and Features

Results

Metal sensitivity assay

The ability of GlpF (overexpressed under IPTG induction) to transport As(III) was tested by an arsenite uptake [http://2009.igem.org/Team:Groningen/Protocols assay]. Also the full accumulation device (BBa_K190038) was tested using this assay. Data and analysis can be found [http://2009.igem.org/Team:Groningen/Project/Accumulation here].

The graphs above represent the result of the metal sensitivity [http://2009.igem.org/Team:Groningen/Protocols#Death_assay assay]. The lines in the graphs represent the average optical density of a construct over time. The graph on the left show that increased As(III) levels inhibit growth and, that as more As(III) is added the lower the plateau is.

The middle graph is from the pLac GlpF construct. The curves are less steep in the log phase compared to WT because of the protein expresion by IPTG induction. In the absence of As(III) the plateau level equals the WT. If arsenite is present the plateaus are lower (OD₆₀₀ <0.8) compared to WT. This is due to As(III) uptake by GlpF.

In the graph on the right we see the curves of low constitutively expressed GlpF and fMT and it shows a similar slope in the log phase compared to pLac GlpF due to protein expression and like WT 0 μM As(III) it has its plateau over OD₆₀₀ 0.9. If arsenite is present the plateaus are lower (OD₆₀₀ <0.8) compared to WT. This is due to As(III) uptake by GlpF. Here the reduced growth is also an indicator for arsenite uptake. It is difficult to see if fMT has an effect because this assay can not show where the arsenite is and how fMT interferes with the cells detoxification.

Arsenic uptake assay

Arsenite uptake by E. coli containing pSB1A2-R0010-GlpF (BBa_K190032) was tested using a arsenic uptake assay (according to Kostal 2004) and arsenic concentration determination by ICP-MS (see [http://2009.igem.org/Team:Groningen/Protocols#Metal_uptake_assay_for_E._coliKostal_2004 protocols]). But because of a lack of reproducibility and a unexpected high uptake yield (as can be seen in figure 1), the improved uptake of arsenite by GlpF could not be determined. For further reading on discussion of this data see the [http://2009.igem.org/Team:Groningen/Accumulation wiki]

Figure 1: Uptake of As(III) by E. coli WT, and the strains containing the different parts of the accumulation device. As a control the arsenic uptake of E. coli with ArsR overexpression (as described by Kostal 2004) is also shown.

references:

Kostal 2004

Jan Kostal, Rosanna Yang, Cindy H. Wu, et al (August 2004). "[http://dx.doi.org/10.1128/AEM.70.8.4582-4587.2004 Enhanced Arsenic Accumulation in Engineered Bacterial Cells Expressing ArsR]". Applied and Environmental Microbiology 70(8): 4582–4587