compared to full-length peptide (Fig. 2, compare lane 16 with
18e19). Furthermore, the overall activity compared to the SplBCON
and SplB-COM was significantly enhanced (compare lanes
18e19 and 20e21).
We next applied the principle to TEV protease, one of the most
ubiquitous enzymes used to remove affinity tags that optimally
cleaves the consensus sequence ENLYFQYS [13,14].We constructed
a fusion substrate wherein this sequence was truncated to ENLYFQ,
and placed between the ePDZ-b and OFP components. TEV is able to
cleave when serine at P1’ position is replaced by methionine with
marginal loss of activity [15]. However, steric constraints due to the
tightly folded N-terminal region of OFP could impact negatively on
cleavage. The results show clearly improved cleavage when TEV is
fused to the optimised 4-amino acid truncated ARVCF peptide
(TEV-AP4) (Fig. 3A). Notable cleavage was observed after only
30 min incubation with majority of substrate beng cleaved after
2.5 h. In comparison, TEV protease only showed significant cleavage
after 24 h incubation. Note that both the TEV-AP4 and TEV proteins
used comprise the S219A mutation introduced to reduce autolysis
[8]. A control experiment using a fusion substrate comprising the
full TEV consensus sequence (ENLYFQS) led to equivalent cleavage
by both wild-type TEV and TEV-AP4 (Fig. 3B). The endogenous TEV
protease (minus the S219V mutation) showed even poorer cleavage
when compared to TEV-AP4 for the same substrate (Fig. S1). We
next assayed TEV-AP4 on a fusion substrate where OFP was
replaced with the fibronectin 10Fn3 domain (ePDZ-b-ENLYFQY-
10Fn3). As before, significantly enhanced cleavage was obtained in
very short times when using TEV-AP4 compared to TEV (Fig. 4A).
Given the highly compact 10Fn3 fold [16], it is likely that steric
constraints inhibit cleavage by TEV. Introduction of GGS or GGSGGS
linkers (to yield ePDZ-b-ENLYFQYMGGS-10Fn3 and ePDZ-b-
ENLYFQYMGGSGGS-10Fn3 respectively) resulted in improved
cleavage by TEV, yet this was still considerably less than for TEVAP4
(Fig. 4B,C). As before, endogenous TEV also performed poorly
using this substrate set (Fig. S2).
We next explored whether the enforced-proximity concept was
applicable to conventional on-column cleavage and purification
protocols using histidine tagged proteins. Complete immobilisation
of protease via its histidine tag could reduce turnover during oncolumn
cleavage of a co-immobilised substrate, necessitating use
of increased amounts for efficient cleavage. Addition of 30 mM
imidazole alleviated this constraint, resulting in improved cleavage
efficiencies using histidine tagged ePDZ-b -WELQ-OFP and SplB-AP
proteins (Fig. S3). These conditions were used for the on-column