by inverse PCR of the protein A-WELQ-SplB-6x His tag-DSWV
construct described above with oSN1396 and oSN1397 followed
by intra-plasmid infusion to truncate the number of histidines from
6 to 5.
For the construct with 4 histidines, the original protein AWELQ-
SplB-6x His tag-DSWV ORF was amplified with oSN684 and
oSN1407, followed by infusion into a vector obtained by inverse PCR
using the oligonucleotides oSN1229 and oSN359.
2.5. ePDZ-b -WELQ-protein constructs
The ePDZ-b ORF was PCR-amplified using tandem oligonucleotides
oSN1344 and 1348 and reverse oligonucleotide oSN1386 to
attach a 6x histidine tag to the N terminus of ePDZ-b and a linker
coding for amino acids “WELQM” to the 3’ terminus. The ORF for
LSSmOrange (OFP) was PCR-amplified using oSN1318 and oSN1312,
followed by SOE PCR to create histidine tag-ePDZ-b-WELQ-OFP
expression cassette. This was then inserted by infusion cloning
into pET22b (Kan) vector obtained by inverse PCR using oSN359-
1384.
2.6. ePDZ-b -TEV site-protein constructs
The previously constructed ePDZ-b-WELQ-OFP plasmid was
inverse PCR-amplified with oSN1410 and oSN1315. The OFP ORF
(PCR-amplified using oSN1414-1412) was inserted into this plasmid
by infusion cloning to yield His tag-ePDZ-b-ENLYFQ-OFP. This
plasmid was subsequently PCR-amplified using primers oSN1415
and oSN1416 and the fibronectin 10Fn3 domain ORF (obtained by
amplifying plasmid 10FN3-HA_pET22b with primers oSN 1417 and
1418) inserted via infusion cloning. The resulting His tag-ePDZ-b-
ENLYFQ-10Fn3 expression plasmid was subsequently amplified by
inverse PCR using primer pairs oSN1419/1420 and oSN1419/1421
and products intramolecularly ligated post-phosphorylation to
yield constructs expressing His-tag-ePDZ-b-ENLYFQ-MGGS-10Fn3
and His-tag-ePDZ-b-ENLYFQ-MGGSGGS-10Fn3 respectively.
2.7. Protein expression
All protein expressionwas carried out in E. coli BL21 (DE3) using
IPTG induction of the T7 promoter. Colonies were inoculated
overnight (ON) in 10 mL TY medium supplemented with the
appropriate antibiotic at 37 C with shaking at 180 rpm. The next
day, the cultures were diluted into 800 mL of fresh medium and
allowed to grow at 37 C to an OD600 of 0.5, upon which IPTG
(1 mM final) was added. Induction was allowed to proceed overnight
at room temperature. The next day, cells were pelleted down
and stored at 20 C until further processing.
2.8. Protein purification
Cell pellets obtained as described above were resuspended in
20 mL of PBS-Tween or other buffers as mentioned in the text
followed by lysis by sonication. The lysate was centrifuged at
14800 rpm for 10 min in 50 mL Falcon tubes and the supernatant
was collected. In parallel, a 1 mL FF His-trap column (GE Healthcare)
was washed with 10 mL of the same buffer in which the pellet
was resuspended. After collecting the cell lysate supernatant, it was
run through the column, followed by washing with 10 mL of
washing buffer (the same buffer used for cell pellet resuspension).
Thereafter elution buffer (50 mM Sodium Phosphate þ 300 mM
NaCl þ 500 mM Imidazole) was run through the column and 1 mL
fractions were collected. All other fractions were also collected and
the normalized volumes of the fractions were run on an SDS-PAGE
gel followed by staining by Instant Blue for visualization.