These cells endogenously express NGF receptors TrkA and P75NTR and, when incubated with wt NGF, undergo neuronal differentiation which manifests morphologically as a neurite network. PC12 cells were incubated for 5 days with,50 ng/ml purified wt NGF, NGF-A4 or biotinylated NGF-A4. The last one was purified from the biotinylation reaction using desalting columns before addition to the cell medium. We found that both NGF-A4 and biotinylated NGF-A4 do induce PC12 differentiation to a similar extent of wt NGF, thus proving that the modified neurotrophin retains its biological activity. We next assessed the biotinylation performance of A1 and S6 tags inserted at the N-terminus of TrkA and P75NTR receptors. We previously demonstrated that insertion of the longer full-length ACP tag, at this position, does not hamper TrkA ability to bind NGF. As for P75NTR, its N-terminal region is not involved in an interaction with bound NGF. We used a biotinylation procedure at the surface of living cells similar to what previously reported for the ACP-TrkA construct. A1- and S6- TrkA-EGFP and P75NTR–EGFP constructs were transfected in SH-SY5Y neuroblastoma cells. 24 h posttransfection the cell monolayer was biotinylated adding CoA-biotin and either AcpS or SfpS PPTases in the cell medium. Cells were then lysed and immunoprecipitated using either anti-TrkA or anti-P75NTR antibodies. Samples were loaded on a gel and blotted using Streptavidin-HRP. Figure 3 shows that the A1 tag is specifically biotinylated by AcpS for both receptors ; the same is true for S6 tag reacted with SfpS. Conversely, the ACP-TrkA used as a control is equally biotinylated by the two PPTases. In general, we found the A1 tag to be less efficiently labeled than the S6 tag for the same construct, especially in the case of TrkA where A1-TrkA is biotinylated about 60% less than S6-TrkA. These data prompted us to use the combination of S6-TrkA and A1-P75NTR in subsequent experiments. Taken together, these data confirm for TrkA and P75NTR in living cells, the properties of orthogonal labeling shown for A1 and S6 tags in previous in vitro studies. Furthermore, as this procedure only allows the biotinylation of the receptor pool exposed at the cell surface, our data suggest that insertion of A1 and S6 tags downstream the signal peptide of TrkA and P75NTR receptors does not inhibit their translocation at the cell membrane. We next examined whether the use of A1 and S6 tags allows the simultaneous fluorolabeling of single molecules of TrkA and P75NTR receptors when coexpressed in the same cell. SH-SY5Y cells were co-transfected with the inducible S6-TrkA and A1-P75NTR constructs. A control transfection with either construct alone was also performed. Transgene expression was then induced using a low dose of doxycycline. Cells were subjected to a BAY-60-7550 439083-90-6 sequential dual-color staining procedure, as outlined in Fig. 4A, in order to label receptors exposed at the cell surface. In more detail, the exposed A1 tag was first biotinylated using AcpS enzyme; A1-P75NTR construct was then coupled to S-Qdot525. In the next step, exposed S6 tag was biotinylated using SfpS enzyme; S6-TrkA construct was finally coupled to SQdot655.