The lactone bond in the lariat was susceptible to hydrolysis, which produced a linear unconstrained peptide. The fraction of L2 lariat that contained an intact lactone bond following purification from E. coli was calculated to be 46%. Lariat formation was sufficient for the Y2H assay but improving lariat stability would increase the concentration of lariat within cells and allow the detection of weaker lariat interactions. It would also increase the probability that peptides isolated with the Y2H assay depend on the lariat for their interaction. Stabilization of the lactone-cyclized lariat is important primarily in downstream applications where it would make it easier to purify and store lariats. Enhanced lariat stability would also facilitate lariat purification and storage.
4.2.2.1 Rationale for constructing mutations
Rational and combinatorial approaches were used to generate mutant lariats with enhanced lactone bond stability. For both approaches, we chose the amino acid positions to mutate based on previous studies, which identified amino acids that stabilized the branched intermediate or on structural studies that revealed amino acid positions that are close to the catalytic site. Both structural and mechanistic studies on intein-mediated protein splicing have identified a variety of mutations that result in the accumulation of a branched intermediate, which is analogous to the lariat intermediate in cyclic peptide intein processing (Figure 4.21).
Figure 4.21 | Intein-mediated peptide splicing
(a) Intein-mediated cyclic peptide production. (b) Intein-mediated ligation. Note the lariat and branched intermediate structures are analogous structures for the two splicing mechanisms.
Mutational studies have been performed on the following three amino acids identified to be essential for intein processing: cysteine at position IN+1/A1, which is essential for the N-S
acyl shift; serine at position IC+1/G8, which is essential for tranesterification; and asparagine at
position IC-1/G7, which is essential for asparagine cyclization. Mutational studies have also
been performed on many amino acids within the conserved blocks (A, B, F, and G) along with several amino acids outside of these conserved blocks (Kawasaki et al., 1997; Chen et al., 2002; Ding et al., 2003; Sun et al., 2005). All possible mutations within the intein would result in millions of different combinations, which represent a sequence space larger than we were able to study. Thus, we focused our study on amino acids at positions IC-2/G6, IC-1/G7, and
B11.
An accumulation of branched intermediate is observed when asparagine at position IC- 1/G7 amino acid is not mutated. This is surprising since asparagine is the key residue for the
third step in intein processing: asparagine cyclization. If branched intermediate is accumulating then asparagine cyclization must be being blocked by mutations at an alternative site. IC-2/G6
was selected as an alternative site to mutate since histidine at this position has previously been shown to be important for lactam peptide cyclization (Scott et al., 1999; Scott et al., 2001), suggesting IC-2/G6 has a role in allowing the intein to process completely. Since the IC-2/G6 is
important for cyclization, mutating this amino acid could block asparagine cyclization. So histidine at position IC-2/G6 was mutated to aspartic acid, asparagine, and leucine while
maintaining asparagine at IC-1/G7 to determine if the lariat could be stabilized by IC-2/G6
mutations.
The IC-1/G7 position was selected since it has been shown previously to contribute to the
stability of the branched intermediate (Kawasaki et al., 1997). Lysine was selected since previous studies had identified that lysine mutations at IC-1/G7 resulted in the accumulation of a
branched intermediate (Kawasaki et al., 1997). Tyrosine was substituted at position IC-1/G7
since it is large and aromatic and we predicted it would have some effect on splicing. Aspartic acid and glutamine are found at IC-1/G7 in a very small percentage of naturally occurring inteins
(Perler, 2002) and were also substituted at position IC-1/G7 in place of asparagine. Aspartic acid
and glutamine are potentially capable of undergoing cyclization just like asparagine but are likely to undergo cyclization at a slower rate, thus they are interesting substitutions since they will mimic asparagine but potentially halt the intein at the lariat.
Crystal structures of several inteins including the Ssp DnaE intein have been solved (Chen et al., 2002; Ding et al., 2003; Sun et al., 2005). Using these structures, we identified an uncharacterized amino acid at position B11 whose role in intein processing has not been examined. B11 is located in the IN domain and is normally arginine. The B11 amino acid was
selected based on the proposed role in asparagine cyclization (Sun et al., 2005) (Figure 4.22). By disrupting this charge relay system we can potentially block asparagine cyclization without mutating the asparagine at (IC-1/G7), which has previously been shown to stabilize the branched
intermediate (Kawasaki et al., 1997). We mutated arginine at position B11 to glutamic acid, tyrosine, and leucine, which represent a range of chemical functionality. Table 4.2 lists the complete set of mutations analyzed.
Figure 4.22 | Role of the B11 position in asparagine cyclization
The proposed role of the B11 arginine in the charge relay system of asparagine cyclization, which is resolved by a tetrahedral intermediate and oxyanion binding site, based on the crystal structure of the Ssp DnaE intein (Sun et al., 2005). IC represents the C-terminal domain of the
intein. Arrows represent the proposed nucleophilic attacks. N HN + Tyr (IN-1) His (F13) H H O H IC-Domain C-Extein N H N H O O OH HN O S N H Cys (IC+1/G8) N-Ex tein Arg (B11) H HN HN NH2 Asn (IC-1/G7) N HN + Tyr (IN-1) His (F13) H H O IC-Domain HNN C-Extein H O O OH HN O S N H Cys (IC+1/G8) N-Ex tein Arg (B11) H HN HN NH2 Asn (IC-1/G7) H +
Table 4.2 | Rational mutations tested to stabilize the lariat
The original mutation used to create the lariat in this thesis is represented by Mutant ‘0’. An additional 29 mutations were constructed to analyze their effect on lariat processing and stability. Amino acids are represented by the single letter amino acid code.
Mutant G6 G7 B11 Mutant G6 G7 B11 Mutant G6 G7 B11
0 H A R 10 H Q L 20 A Y L 1 N N Y 11 H Q E 21 A Y E 2 N N R 12 H A Y 22 A K Y 3 N N L 13 H A L 23 A K R 4 N N F 14 H A E 24 A K L 5 L N Y 15 D N Y 25 A K E 6 L N R 16 D N R 26 A D Y 7 L N L 17 D N L 27 A D R 8 L N E 18 D N E 28 A D L 9 H Q R 19 A Y R 29 A D E
4.2.2.2 Analysis of lariat stability
Initially, we optimized lariat expression and examined the effects of induction time and the time between collection and analysis on lariat stability. Lariat stability was analyzed using liquid chromatography directly coupled to electrospray ionization time-of-flight mass spectrometry (LC/MS). The LC step was used to desalt and separate the intein products based on hydrophobicity. The lariats were quantified using total ion count (TIC). The peaks eluted from the LC represented different intein fragments and were first integrated to generate a mass/charge (m/z) versus peak intensity spectrum. The charged spectrum was processed using maximum entropy calculations (MaxEnt) to deconvolute the spectrum into its component molecular weight proteins and their abundance. The MaxEnt data was used to calculate ‘% lariat’, ‘% processed’, and ‘% total lariat’.
% lariat
The “% lariat” represents hydrolysis or the stability of the lactone bond and was the percentage of intact lariat in the sample compared to the hydrolyzed product. The % lariat was calculated by dividing the area under the lariat peak by the sum of the areas under the lariat and hydrolyzed lariat peaks. The hydrolyzed lariat differs from the lariat by the addition of H2O,
which increased its mass by 18 Da.
% processed
“% processed” represents intein processing and was the percentage of lariat that was produced in the sample. It does not differentiate between lariat and hydrolyzed lariat. Percent processed is defined as the ratio of the area under the peak representing the IN domain to the
total amount of intein represented by the sum of the area under the peak representing the full length unprocessed protein and the IN domain.
% total lariat
The “% total lariat” represents the total amount of lariat produced in a cell and is a function of % lariat and % processed. It is defined by the % processed multiplied by the % lariat that is not hydrolyzed.
We first established the optimal conditions for expression and the effect of lariat storage on stability on the L2 lariat. We induced cultures expressing the lariat for 2, 4, 7, or 18 hours. The samples were stored at 4 °C and then analyzed again 20 hours later. The induction time ! %Lariat = Lariat Lariat + Hydrolyzed x100% ! %Pr ocessed = INDomain INDomain + Unprocessedx100%
effected the lariat stability with ~ 10% loss of lariat between the samples induced for 2 hours and the samples induced for 18 hours (Figure 4.23). However, the amount of time a sample spent at 4 oC before processing had little effect. After 20 hours, ~ 1 - 3% of the lariat was hydrolyzed (Figure 4.24). Together, these results suggest that the lariat was being hydrolyzed within the cell and that the lariat was relatively stable after purification.
Once we established the optimal conditions for expression and storage, the other mutants were expressed and analyzed under the same conditions. The mutants were sorted by % lariat, % processed, or % total lariat (Figure 4.25- Figure 4.27).
Figure 4.23 | Effect of induction time on lariat hydrolysis
Lariat expression was induced with IPTG for the time indicated and then purified using Ni2+- NTA beads and analyzed immediately after purification by LC/MS. The % lariat was calculated by dividing the area under the lariat peak by the sum of the areas under the lariat and hydrolyzed lariat peaks and plotted versus induction time. Error bars represent the standard deviation of three independent experiments.
Time (hr)
% Lar
ia
Figure 4.24 | Effect of storage time on lariat hydrolysis
Lariat expression was induced for 2, 4, 7, or 18 hours and purified using Ni2+-NTA beads and analyzed immediately after purification by LC/MS (Black bars) and again after sitting for 20 hours at 4 °C (Grey bars). The % lariat was calculated by dividing the area under the lariat peak by the sum of the areas under the lariat and hydrolyzed lariat peaks. Error bars represent the standard deviation of three independent experiments.
% Lar
ia
t
Figure 4.25 | Lariat mutations sorted by % lariat
Lariat expression was induced with IPTG for 2 hours and then purified using Ni2+-NTA beads
and analyzed by LC/MS. Mutant lariats are defined by the three amino acid mutations at positions (G6/G7/B11). The mutations are compared to the HAR lariat, which is shown twice; one point representing a 2 hr induction and a second point representing an 18 hr induction. The % lariat (White bars) was calculated by dividing the area under the lariat peak by the sum of the areas under the lariat and hydrolyzed lariat peaks. The % processed (Grey bars) was calculated by dividing the area under the peak representing the IN domain by the sum of the area under the
peak representing the full length unprocessed protein and the IN domain. The % total lariat
(Black bars) was calculated by multiplying the % processed by the % lariat. Mutations are sorted in order of highest % lariat to lowest % lariat.
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% A YE DNR NNR AYR LNR HQR AD Y NN L AD L LN L H A Y AK L LNE HQ L LN Y AK Y
AKR HAR NNE AY
L NN Y ADR HA L DN L
AKE HAR HQE ADE HAE DNE DN
Y % Lariat % Processing % Total Lariat Mutants (G6/G7/B11) % Lar ia t 2 hr 18 hr
.
Figure 4.26 | Lariat mutations sorted by % processing
Lariat expression was induced with IPTG for 2 hours and then purified using Ni2+-NTA beads and analyzed by LC/MS. Mutant lariats are defined by the three amino acid mutations at positions (G6/G7/B11). The mutant lariats are compared to the HAR lariat, which is shown twice; one point representing a 2 hr induction and a second point representing an 18 hr induction. The % lariat (White bars) was calculated by dividing the area under the lariat peak by the sum of the areas under the lariat and hydrolyzed lariat peaks. The % processed (Grey bars) was calculated by dividing the area under the peak representing the IN domain by the sum
of the area under the peak representing the full length unprocessed protein and the IN domain.
The % total lariat (Black bars) was calculated by multiplying the % processed by the % lariat. Mutations are sorted in order of highest % processing to lowest % processing.
Mutants (G6/G7/B11) % P roc essing 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% HAR AD Y ADE ADR AD L HAR LNR HQR LN Y
HAE AKR LNE AKE AYE AK
L NNE AK Y H A Y A YR HQE HA L HQ L LN L NN Y A Y L NNR NN L DN Y DNE DNR DN L % Lariat % Processing % Total Lariat 18 hr 2 hr
Figure 4.27 | Lariat mutations sorted by % total lariat
Lariat expression was induced with IPTG for 2 hours and then purified using Ni2+-NTA beads and analyzed by LC/MS. Mutant lariats are defined by the three amino acid mutations at positions (G6/G7/B11). The mutant lariats are compared to the HAR lariat, which is shown twice; one point representing a 2 hr induction and a second point representing an 18 hr induction. The % lariat (White bars) was calculated by dividing the area under the lariat peak by the sum of the areas under the lariat and hydrolyzed lariat peaks. The % processed (Grey bars) was calculated by dividing the area under the peak representing the IN domain by the sum
of the area under the peak representing the full length unprocessed protein and the IN domain.
The % total lariat (Black bars) was calculated by multiplying the % processed by the % lariat. Mutations are sorted in order of highest % total lariat to lowest % total lariat.
Highly stable mutants were isolated that showed > 60% lariat by direct MS analysis. The actual amount of lariat in vivo was likely higher, since MS processing has been shown to convert lariat into hydrolyzed lariat. Previously, we found the wild-type lariat was present at ~ 25% of the total lariat by direct MS analysis whereas the amount of lariat present prior to MS analysis was 46%.
The data was analyzed by plotting the % processing against the % lariat on a double log scale to allow all the mutations to be visualized (Figure 4.28). Since the percent total lariat was
Mutants (G6/G7/B11) % Total Lar ia t 18 hr 2 hr 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% AD Y AD L LNR HQR AYE ADR HAR LN Y
LNE HAR AYR AKR ADE AK
L H A Y AK Y
NNR NNE AKE HAE LN
L HQ L HA L A Y L NN L NN Y HQE DNR DN L DN Y DNE % Lariat % Processing % Total Lariat
a function of both % lariat and % processing, diagonal lines on this plot indicate the threshold for 20, 30, 40, and 50% total lariat. Thus, mutants sharing a diagonal line will have the same percentage of total lariat. Mutants that have better stability and higher processing were found in the upper right of the plot and those with poor processing and stability were found in the lower left of the plot. This plot was used to create vector maps, where mutants were connected by lines from the lowest stability (% lariat) to highest stability (% lariat). These vector maps can be compared directly with other mutations to identify maps where mutations result in similar patterns. The direction of the vector, not the magnitude of change, is important in these maps since they were plotted on a double log scale. For example, in Figure 4.29, the mutant series (DNX) where the B11 position is varied (X) and the IC-2/G6 (D) and IC-1/G7 (N) positions
are fixed was plotted. The resulting vector map can be visually compared to other vector maps to identify mutants that behave similarly.
Figure 4.28 | Mutants summary map
Comparison of the percent processing versus percent lariat for mutants, plotted on log scales. The total lariat for 20%, 30%, 40% and 50%, which is calculated by multiplying the percent lariat by the percent processing is shown to assist in comparing mutants. Mutations are named by their position (G6/G7/B11).
Figure 4.29 | Vector maps showing the G7-B11 interaction
(a) Vector map showing % lariat versus % processing for the G6/G7/B11 mutants, where G6 is aspartic acid (D), G7 is asparagine (N), and B11 is arginine (R), leucine (L), tyrosine (Y), or glutamic acid (D). Mutants are connected with lines from the lowest to the highest percent lariat to produce a vector map. Each mutant is abbreviated as three letters according to its G6/G7/B11 position. “X” represents the amino acid that is varied and is described by the letter next to the dot in the map. (b) Vector maps of XNX and XQX mutants. (c) The conserved pattern of the vector maps shown in (b). (d) Vector maps of HAX, AKX, and ADX mutants. (e) Vector map of the AYX mutant.
4.2.2.3 Mutation of the IC-2/G6 amino acid
XNR, XNL, XNY, and XNE mutations were used to determine the effect of mutations at position IC-2/G6 on lariat stability and processing, where X is leucine, asparagine, or aspartic
acid (Figure 4.26). In these mutants, the IC-2/G6 amino acid had a drastic effect on processing.
Leucine had the best processing, followed by asparagine. Aspartic acid almost completely blocked processing in all cases. As suggested by previous studies, the IC-2/G6 amino acid had
the greatest effect on processing rather than on stability, as noted by the grouping of the IC-2/G6
amino acids when sorted by % processing (Figure 4.26). Construction of more mutants where the IC-2/G6 amino acid is paired with alternative IC-1/G7 amino acids and B11 amino acids will
be required to determine how the IC-1/G6 amino acid influences processing and stability when
amino acids other than asparagine are substituted at the IC-1/G7 position.
4.2.2.4 Effect of asparagine to lysine mutation at position IC-1/G7 on lariat processing
Asparagine at position IC-1/G7 is essential for asparagine cyclization in the intein-
mediated cyclization reaction. The asparagine side chain undergoes cyclization to cleave the IC
domain from the lariat and produce a lactone peptide. It is essential to block asparagine cyclization either by directly mutating the IC-1/G7 amino acid or indirectly by mutating another
amino acid to produce the lariat. In the initial lariat library (pIL-R7), we mutated the IC-1/G7
asparagine to alanine to block asparagine cyclization. In the standard intein reaction, the branched intermediate accumulates when asparagine at position IC-1/G7 is mutated to lysine
(Kawasaki et al., 1997). Not all mutations at position IC-1/G7 result in an accumulation of
branched intermediates, for example serine or alanine do not (Chong et al., 1996). Interestingly, mutation of asparagine at position IC-1/G7 to alanine leads to the accumulation of
branched intermediate if cysteine at position IC+1/G8 is also mutated to serine (Chong et al.,
1996), which is the case for the pIL-R7 lariat intein library. Based on these results, we tested the effect of mutating asparagine to lysine at position IC-1/G7. Lysine was tested in
combination with tyrosine, arginine, leucine, and aspartic acid at the B11 position, but only with alanine at the IC-2/G6 position. Surprisingly, the lysine mutation at position IC-1/G7 in all
mutation combinations tested was similar to our initial construct and did not result in a significant improvement in stability or processing. This suggested that mutations that stabilized the branched intermediate may not apply to the lariat intermediate or that the lysine mutation at IC-1/G7 had no effect when IC-2/G6 was alanine.
4.2.2.5 The G7 position interacts with the B11 position
In the absence of histidine at IC-2/G6, it has been suggested that arginine at position B11
can assist in asparagine cyclization by hydrogen bonding to the asparagine carbonyl oxygen at position IC-1/G7 (Ding et al., 2003). B11 is predominately lysine or arginine when IC-2 (G6) is
not histidine (Perler, 2002). Currently, there are no mutagenic studies on the role of arginine at position B11.
When the vector maps of mutations with IC-1/G7 as either asparagine or glutamine were
compared, they showed a similar trend in how they responded to B11 mutations (glutamic acid, tyrosine, leucine, and arginine) (Figure 4.29) (note the HQY and AYY mutations are absent since they were not successfully cloned). Regardless of the IC-2/G6 amino acid (histidine,
asparagine, leucine, or aspartic acid), when B11 was glutamic acid or tyrosine, the lariat showed the lowest stability. When B11 was mutated to leucine, the lariat stability increased but the lariat processing decreased compared to when B11 was glutamic acid or tyrosine (L is always to the left and up of Y or E on the vector maps in Figure 4.29). When B11 was arginine lariat stability was increased compared to when B11 was leucine (R is always up and to the right of L on the vector maps in Figure 4.29). This data indicates that there was an interaction between the IC-1/G7 and B11 amino acid, but less of an interaction between the IC-2/G6 and B11
amino acids since the general shape of the vector maps did not change despite radical changes in the IC-2/G6 amino acid.
The vector maps also indicate that the IC-1/G7-B11 interaction could be further
optimized by mutating the B11 amino acid from arginine to another positively charged amino acid such as lysine or histidine. Hydrophobic amino acids (leucine) reduced processing, and