The transmembrane (TM) anchors of many signaling receptors actually play critical roles in receptor signaling, and the diversity of mechanism with which the TM regions can promote signaling is beyond the traditional views in receptor biology. 

Oligomer labeling (OG-label), the soluble crosslinkable protein (SCP) used is a small protein named GB1 (M.W. = 8.4 kDa). Its N-terminus is linked to a TriNTA molecule via a crosslinker to form the TriNTA-GB1 conjugate. The target TM protein to be examined has a His6-tag. The TriNTA molecule has extremely high binding affinity to His6-tag sequence (20 ± 10 nM), which can strongly attach GB1 to the individual protomers of the TMD oligomer in bicelles.

TriNTA was synthesized by Medicilon.

Reference:

Qingshan Fu, et al. Structure determination protocol for transmembrane domain oligomers. Nat Protoc. 2019 Aug;14(8):2483-2520. doi: 10.1038/s41596-019-0188-9.