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Revision as of 19:02, 28 September 2024


CB2/CB2A HfsL Glycosyltransferase, 6xHis tag for purification

Introduction

Usage and Biology

Caulobacter crescentus is a common freshwater gram-negative oligotrophic bacterium of the clade Caulobacterales. Its distinguishing feature is its dual lifestyle. Initially, C. crescentus daughter cells are in a “swarmer” cell phase, which has a flagellum, enabling them to perform chemotaxis. After the motile phase, they differentiate into “stalked” cells. This phase features a tubular stalk with an adhesive structure holdfast, allowing them to adhere to surfaces and perform cell division.

Caulobacterales synthesize a polysaccharide-based adhesin known as holdfast at one of their cell poles, enabling tight attachment to external surfaces. It is established that holdfast consists of repeating identical units composed of multiple monomers. Current literature agrees that in Caulobacter crescentus, these units form tetrads composed of glucose, an unidentified monosaccharide (either N-mannosamine uronic acid or xylose), N-acetylglucosamine, and N-glucosamine. These units are polymerized and exported to the outer membrane of the cell, where they function as anchors, securing the bacterium to a surface.

The C. crescentus holdfast is produced via a polysaccharide synthesis and export pathway similar to the group I capsular polysaccharide synthesis Wzy/Wzx-dependent pathway in Escherichia coli.

The holdfast synthesis (hfs) genes include those encoding predicted glycosyltransferases, carbohydrate modification factors, and components of a wzy-type polysaccharide assembly pathway.

HfsL in particular is responsible for N-acetyl-D-glucosamine transfer to the acceptor molecule.


This part also has a non 6xhis-tagged variant BBa_K5246012.

Sequence and Features


Assembly Compatibility:
  • 10
    COMPATIBLE WITH RFC[10]
  • 12
    COMPATIBLE WITH RFC[12]
  • 21
    COMPATIBLE WITH RFC[21]
  • 23
    COMPATIBLE WITH RFC[23]
  • 25
    INCOMPATIBLE WITH RFC[25]
    Illegal NgoMIV site found at 265
    Illegal NgoMIV site found at 408
    Illegal NgoMIV site found at 639
  • 1000
    COMPATIBLE WITH RFC[1000]



Bioinformatic analysis

CDD analysis showed specific hits in glycosyl transferase family 2. This diverse family transfers sugar from UDP-glucose, UDP-N-acetyl-galactosamine, GDP-mannose, or CDP-abequose to a range of substrates. Protein BLAST further supports these findings and suggests that HfsL is most likely a family 2 glycosyltransferase, which has a domain very similar to the poly-beta-1,6-N-acetyl-D-glucosamine synthase domain of biofilm PGA synthase.

DeepTMHMM analysis suggests that the protein is likely globular and positioned on the inner side of the cell membrane. The AlphaFold3 structure provides additional evidence supporting its globular shape. A pTM score above 0.5 suggests that the predicted overall structure may closely resemble the true protein fold, while ipTM indicates the accuracy of the subunit positioning within the complex. Values higher than 0.8 represent confident, high-quality predictions (Fig.1).

To sum up, HfsL is most probably a globular family 2 glycosyltransferase, responsible for N-acetyl-D-glucosamine transfer to the acceptor molecule, as is further verified by existing research. [1][2][3]

hfsl.png
Fig. 1. AlphaFold 3 structure showing

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Experimental characterization

Protein expression

We chose the BL21(DE3) strain for adjustable and efficient expression of target proteins since the system's proteins were best expressed in this strain. Given the lack of time, we went with conditions optimized beforehand in earlier experiments for the whole pathway expression: temperature of 37°C, induction with 0.5 mM IPTG concentration, and expression for 3 hours.

After SDS-PAGE gel analysis, we concluded that we successfully expressed all HfsG, HfsH, HfsJ, HfsK, and HfsL proteins from C. crescentus CB2 and CB2A strains. We noticed that HfsJ and HfsL glycosyltransferases were visible in lower quantities compared to the other proteins. Both of these protein expression conditions need to be further investigated and optimized.

HfsL is clearly visible on the right side of the gel (Fig. 2).


Fig. 2. 12% SDS-PAGE analysis of C. crescentus CB2 strain proteins in BL21(DE3) before expression and after induction at 0.5 mM IPTG concentrations for 3 hours at 37°C. M - molecular weight ladder in kDa, Pageruler Unstained Protein Ladder, 26614 (Thermo Scientific).

Table 1. C. crescentus protein sizes in kDa

Protein Name Size (kDa)
HfsG 34
HfsH 27.9
HfsJ 34.7
HfsK 43.3
HfsL 33.3
===References=== 1. Hershey, D.M., Fiebig, A. and Crosson, S. (2019) ‘A genome-wide analysis of adhesion in Caulobacter crescentus identifies new regulatory and biosynthetic components for holdfast assembly’, mBio, 10(1). doi:10.1128/mbio.02273-18.
2. Chepkwony, N.K., Hardy, G.G. and Brun, Y.V. (2022) ‘HFAE is a component of the holdfast anchor complex that tethers the holdfast adhesin to the cell envelope’, Journal of Bacteriology, 204(11). doi:10.1128/jb.00273-22.
3. Chepkwony, N.K., Berne, C. and Brun, Y.V. (2019) ‘Comparative analysis of ionic strength tolerance between freshwater and marine Caulobacterales adhesins’, Journal of Bacteriology, 201(18). doi:10.1128/jb.00061-19.