For instance, DNase colicin E2 attached to the cell surface is susceptible to proteolytic cleavage by OmpT into a 50 kDa fragment, and this has an antagonistic effect on colicin import, as in the case of RNase colicins. The presence of MAF, detected at the cell surface of DNasecolicin treated cells is not necessary for colicin translocation through the outer membrane or the inner membrane. The MAF attached to the outer membrane is detected only after the PK treatment. The PK-resistant MAF structure includes both the receptor binding and DNase domains. This indicates that in the absence of PK, numerous DNase colicin molecules, which have not been imported or are “waiting” to be imported, are still strongly associated with the receptor BtuB, when the processed and translocated DNase domain was detected in the cytoplasm. This result is supported by previous observations, notably derived from colicin competition experiments, which suggested that a partial colicin A molecule or the Tand R-domains of colicin E2 remain in contact with BtuB throughout the import process. The novelty of our findings is the demonstration of the stable colicin association with the outer membrane BtuB, which concerns whole colicin molecules not yet translocated into the periplasm, as judged by the presence of the catalytic domain present in MAF, rather than colicin molecules whose DNase domains have already penetrated inside the target cells, as suggested previously. Such a stable association presumably facilitates the penetration of incoming colicin molecules into target cells. A MAF-like peptide was detected with RNase colicin E3, but only after a very mild PK treatment of colicin E3 treated cells. The existence of similar MAF, derived from either RNase or DNase colicin molecules, is not really surprising, since the overall translocation mechanism across the outer membrane is known to be very similar for E-type nuclease colicins. MAF resistance to PK is presumably due to intrinsic properties of the MAF peptide sequence, including the RNase or DNase domain and amino-acid changes grouped around the C-terminus of the R-domains. Thus, changes in the coiled coil structure of R-domains that binds BtuB is possibly the key element of the selective resistance of the colicins E2 and E3 MAF to PK. In situ studies on the mechanisms of cell fate regulation in local microenvironments has gained considerable interest in the development of cell based therapies for disease and regeneration. These studies are very often complemented with bioluminescence imaging assays that yield valuable information on cell fate and behavior in a dynamic microenvironment. Likewise, specific niche components have been screened for their contribution to therapy outcome, including the matrix elasticity, presence of soluble and matrix-bound chemical agents in biomaterials, targeted and sustained release of cytokines from transplanted cells.