> ## Documentation Index > Fetch the complete documentation index at: https://docs.adaptyvbio.com/llms.txt > Use this file to discover all available pages before exploring further. # Construct & Tag Design > The standard tag construct we append to every binder for expression, purification, and immobilization on the BLI or SPR sensor, and why it stays clear of the binding site. You submit your binder as a bare sequence (no tags needed). Before expression, we append a standard tag construct that handles three jobs at once: expression quantification, purification, and immobilization on the assay instrument. This page shows exactly what we add, where it goes, and how we keep it away from the binding interface. ## The standard construct By default we append the following to the C-terminus of your binder, connected by flexible GGGS linkers:
Your binder
your design
GGGS
split-GFP11
expression
GGGS
Twin-Strep-tag
purify + immobilize
**Full sequence appended to your protein** (GGGSGGGS linker, then the tag):
GGGSGGGSRDHMVLHEYVNAAGITGGGSGGGSGGSAWSHPQFEKGGGSGGGSGGSAWSHPQFEK
split-GFP11 Twin-Strep-tag (two Strep-tag II copies) unhighlighted = GGGS linkers
| Component | Sequence | Purpose | | - | - | - | | GGGS linkers | `GGGSGGGS…` | Flexible spacers that decouple the tag from the folded domain | | split-GFP11 | `RDHMVLHEYVNAAGIT` | Expression quantification via split-GFP complementation (not used for purification) | | Twin-Strep-tag | `GGSAWSHPQFEKGGGSGGGSGGSAWSHPQFEK` | Purification and immobilization handle (two tandem Strep-tag II joined by a GGGSGGGS linker) | The Twin-Strep-tag does double duty: it is how we purify the protein and how we capture it onto the sensor. On our platform, capture is the purification step. ## Why these tags * **Twin-Strep-tag:** a high-affinity, well-behaved handle for Strep-Tactin XT. We use it both to purify the binder and to immobilize it on the BLI or SPR sensor, so a single tag covers the whole workflow. * **split-GFP11:** a small fragment that reconstitutes fluorescence with a complementary GFP1-10 fragment, letting us quantify expression without a separate readout. See [Split-GFP quantification](/wiki/technologies/split-GFP) for the method. We do not cleave purification tags. Instead we choose placement (or follow your preference) so the tag stays clear of the binding site, which is what the structures below are for. ## Tag placement **C-terminal is the default.** In our hands it gives the most consistent expression across protein types. You can request N-terminal placement instead if the C-terminus is functionally important for your design; just flag it at submission. The construct is designed so the tag sits away from the binding interface regardless of scaffold. The predicted structures below show the tag position relative to the paratope across representative protein types. In each: binder green, target blue, and the C-terminal tag gold, extending away from the binding site. De novo binder against IL-7Ra with C-terminal tag extending away VHH nanobody against Nipah-G with C-terminal tag extending away scFv against EGFR with C-terminal tag extending away ## How the tag is used in the assay We measure binding kinetics by BLI on the Gator Bio Pro or SPR on the Carterra LSA XT. On both, the Twin-Strep-tag is the capture handle: it immobilizes your binder as the ligand on a Strep-Tactin surface (a biosensor tip for [BLI](/wiki/technologies/BLI), or the sensor chip for [SPR](/wiki/technologies/SPR)), while the target is delivered in solution as the analyte. 1. **Capture.** Your Twin-Strep-tagged binder is captured onto the Strep-Tactin surface. This capture step also serves as the purification, so no separate purification is required on our platform. 2. **Kinetics.** The target is presented in solution across a concentration series, and real-time association and dissociation yield kon, koff, and KD. ## See also * [Split-GFP quantification](/wiki/technologies/split-GFP): how we use the split-GFP11 fragment to quantify expression. * [Affinity Tag Purification](/wiki/technologies/affinity-tag-purification): the purification handle in more detail. * [BLI](/wiki/technologies/BLI) and [SPR](/wiki/technologies/SPR): the binding assays this construct is built for. This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.