In vitro incorporation of synthetic peptides into cells

Comparing the intracellular mobility of fluorescent proteins following in vitro expression or cell loading with streptolysin-O

The application of fluorescent proteins in live cells has greatly improved our ability to study molecular mobility, which both reflects molecular function in live cells and reveals the properties of the local environment. Although measuring molecular mobility with fluorescent fusion proteins is powerful and convenient, certain experiments still require exogenous macromolecules to be loaded into cells. Cell viability provides a rough gauge of cellular damage following membrane permeabilization, but it is unknown how permeabilization will affect intracellular mobility. We have used fluorescence correlation spectroscopy to measure the intracellular dynamics of the enhanced green fluorescent protein (EGFP) in living human embryonic kidney (HEK) cells under conditions where the EGFP is either expressed or loaded using streptolysin O (SLO) permeabilization to determine how permeabilization effects mobility. We found that purified EGFP loaded with SLO has the same mobility as the expressed EGFP, while the mobility of the expressed EGFP after SLO permeabilization treatment becomes slightly slower. Our results indicate that SLO permeabilization is often accompanied by the loss of cellular soluble proteins to the surrounding medium, which explains the apparent decrease in diffusion rates following treatment. These measurements are also relevant to the role of molecular crowding in the intracellular mobility of proteins.

Antennapedia/HS1 chimeric phosphotyrosyl peptide: conformational properties, binding capability to c-Fgr SH2 domain and cell permeability

With the aim of interfering with the signaling pathways mediated by the SH2 domains of Src-like tyrosine kinases, we synthesized a tyrosyl-phospho decapeptide, corresponding to the sequence 392-401 of HS1 protein, which inhibits the secondary phosphorylation of HS1 protein catalyzed by the Src-like kinases c-Fgr or Lyn. This phospho-peptide was modified to enter cells by coupling to the third helix of Antennapedia homeodomain, which is able to translocate across cell membranes. Here we present CD and fluorescence studies on the conformational behavior in membrane-mimicking environments and on lipid interactions of Antennapedia fragment and its chimeric phosphorylated and unphosphorylated derivatives. These studies evidenced that electrostatic rather than amphiphilic interactions determine the peptide adsorption on lipids. Experiments performed with recombinant protein containing the SH2 domain of c-Fgr fused with GST and with isolated erythrocyte membranes demonstrated that the presence of the N-terminal Antennapedia fragment only slightly affects the binding of the phospho-HS1 peptide to the SH2 domain. In fact, it has been shown that in isolated erythrocyte membranes, both phospho-HS1 peptide and its chimeric derivative greatly affect either the SH2-mediated recruitment of the c-Fgr to the transmembrane protein band 3 and the following phosphorylation of the protein catalyzed by the Src-like kinase c-Fgr. The ability of the chimeric phospho-peptide to enter cells has been demonstrated by confocal microscopy analysis.

Delivery of proteins into living cells by reversible membrane permeabilization with streptolysin-O

The pore-forming toxin streptolysin O (SLO) can be used to reversibly permeabilize adherent and nonadherent cells, allowing delivery of molecules with up to 100 kDa mass to the cytosol. Using FITC-labeled albumin, 10(5)-10(6) molecules were estimated to be entrapped per cell. Repair of toxin lesions depended on Ca(2+)-calmodulin and on intact microtubules, but was not sensitive to actin disruption or to inhibition of protein synthesis. Resealed cells were viable for days and retained the capacity to endocytose and to proliferate. The active domains of large clostridial toxins were introduced into three different cell lines. The domains were derived from Clostridium difficile B-toxin and Clostridium sordelli lethal toxin, which glycosylate small G-proteins, and from Clostridium botulinum C2 toxin, which ADP-ribosylates actin. After delivery with SLO, all three toxins disrupted the actin cytoskeleton to cause rounding up of the cells. Glucosylation assays demonstrated that G-proteins Rho and Ras were retained in the permeabilized cells and were modified by the respective toxins. Inactivation of these G-proteins resulted in reduced stimulus-dependent granule secretion, whereas ADP-ribosylation of actin by the C. botulinum C2-toxin resulted in enhanced secretion in cells. The presented method for introducing proteins into living cells should find multifaceted application in cell biology.