Outer membrane proteins (OMPs) of Gram-negative bacteria, are translocated across the cytoplasmic membrane in unfolded form by the SecYEG translocon. In the periplasm, a signal peptidase cleaves off the leader-sequence, before the unfolded OMPs bind to a molecular chaperone, the seventeen-kDa protein, Skp. The properties of the Skp complexes with OMPs are not well characterized and were investigated previously only on the example of outer membrane protein A of E. coli [1]. Here we have used tryptophan fluorescence spectroscopy to examine the interactions of wild-type Skp, which is devoid of tryptophan, with four OMPs of different size and function, namely wild-type OmpA (35 kDa), its transmembrane domain (19.3 kDa), the porin OmpG (32.8 kDa), the barrel domain of the autotransporter NalP (31.8 kDa), and YaeT (88.4 kDa), which is essential for targeting or assembly of OMPs into the outer membrane. In all cases, the Skp-trimer bound in 1:1 stoichiometry. The free energy of complex formation ranged from 42 kJ/mol to 45 kJ/mol for most outer membrane proteins, but was higher for YaeT, 51 kJ/mol. Skp efficiently shielded the tryptophans of OmpA against interaction with acrylamide as determined in fluorescence quenching experiments. Upon addition of lipopolysaachride, tryptophans of OmpA, which has been in complex with Skp, became slightly more accessible to the fluorescence quencher, but were still more shielded than in OmpA in aqueous solution, or in OmpA in presence of lipopolysaccharide, but in absence of Skp, indicating formation of a ternary complex. At high salt concentration, the free energy of binding of Skp to OmpA was reduced, indicating an electrostatic component in binding. [1] Bulieris, P. V., Behrens, S., Holst, O., and Kleinschmidt, J. H. 2003, J. Biol. Chem. 278, 9092-9099.