Rhodococcus jostii RHA1 is a catabolically versatile soil actinomycete that can utilize a wide range of org. compds. as growth substrates including steroids. To globally assess the adaptation of the protein compn. in the membrane fraction to steroids, the membrane proteomes of RHA1 grown on each of cholesterol and cholate were compared to pyruvate-grown cells using gel-free SIMPLE-MudPIT technol. Label-free quantification by spectral counting revealed 59 significantly regulated proteins, many of them present only during growth on steroids. Cholesterol and cholate induced distinct sets of steroid-degrading enzymes encoded by paralogous gene clusters, consistent with transcriptomic studies. CamM and CamABCD, two systems that take up cholate metabolites, were found exclusively in cholate-grown cells. Similarly, 9 of the 10 Mce4 proteins of the cholesterol uptake system were found uniquely in cholesterol-grown cells. Bioinformatic tools were used to construct a model of Mce4 transporter within the RHA1 cell envelope. Finally, comparison of the membrane and cytoplasm proteomes indicated that several steroid-degrading enzymes are membrane-assocd. The implications for the degrdn. of steroids by actinomycetes, including cholesterol by the pathogen Mycobacterium tuberculosis, are discussed. [on SciFinder(R)]