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Both SRC-1 and TIF2 are members of the p160 steroid receptor coactivator family. Genetic analyses have shown that inactivation of TIF2, but not SRC-1, reduces postnatal survival, growth, and male reproductive function. Here, we demonstrate that, through analyses of SRC-1/TIF2 compound mutant mice, SRC-1 can partially compensate for the effects of a loss of TIF2 on mouse survival and growth, whereas SRC-1 and TIF2 are dispensable for primary organogenesis. The highly variable onset of defects observed in TIF2(-/-) testes due to the absence of TIF2 in Sertoli cells, including abnormal spermiogenesis, age-dependent degeneration of seminiferous epithelium, and disorder of cholesterol homeostasis, is uniformly accelerated upon inactivation of SRC-1 alleles in the TIF2 null genetic background, thus demonstrating that TIF2 and SRC-1 can perform redundant functions in Sertoli cells. Massive desquamation of immature germ cells together with an increase in germ cell apoptosis and a decrease in germ cell proliferation may be responsible for the early onset of the severe seminiferous epithelial degeneration observed in SRC-1(+/-)/TIF2(-/-) testes. Interestingly, the overall abnormal features displayed by the SRC-1(+/-)/TIF2(-/-) and SRC-1(-/-)/TIF2(-/-) mutant testes, including spermatid maturation defects, increase in Sertoli cell lipid stores, loss of immature germ cells, and formation of giant multinucleated spermatids, are commonly detected in testes of elderly men, suggesting that deficiencies in molecular pathways involving TIF2 and SRC-1 in Sertoli cells could participate in testicular senescence.

Steroid receptor coactivator-3 (SRC-3) is a coactivator of nuclear receptors in the SRC family as assayed in vitro. Here, we show that mouse SRC-3 is expressed in a tissue-specific fashion and distributed mainly in the oocytes, mammary glands, hippocampus, olfactory bulb, smooth muscle, hepatocytes, and vaginal epithelium. Genetic disruption of SRC-3 in mice results in a pleiotropic phenotype showing dwarfism, delayed puberty, reduced female reproductive function, and blunted mammary gland development. Hormonal analysis indicates that SRC-3 plays a role in both the growth hormone regulatory pathway and the production of estrogen, which may explain the observed phenotypes. These results suggest that the physiological role of SRC-3 is different from that of SRC-1 and prove the diversity among coactivator family members.

The three-dimensional structure of the protein subunits of the reaction center (RC) of Rhodobacter sphaeroides has been determined by x-ray diffraction at a resolution of 2.8 A with an R factor of 26%. The L and M subunits each contain five transmembrane helices and several helices that do not span the membrane. The L and M subunits are related to each other by a 2-fold rotational symmetry axis that is approximately the same as that determined for the cofactors. The H subunit has one transmembrane helix and a globular domain on the cytoplasmic side, which contains a helix that does not span the membrane and several beta-sheets. The structural homology with RCs from other purple bacteria is discussed. A structure of the complex formed between the water soluble cytochrome c2 and the RC from Rb. sphaeroides is proposed.

The energetics of membrane-protein interactions are analyzed with the three-dimensional model of the photosynthetic reaction center (RC) from Rhodobacter sphaeroides. The position of the RC in the membrane and the thickness of the membrane were obtained by minimizing the hydrophobic energy with the energy function of Eisenberg and McLachlan. The 2-fold symmetry axis that relates the L and M subunits is, within the accuracy of 5 degrees, parallel to the normal of the membrane. The thickness of the membrane is estimated to be 40-45 A. Residues that are exposed to the membrane are relatively poorly conserved in the sequences of homologous RC proteins. The surface area of the RC is comparable to the surface areas of water-soluble proteins of similar molecular weight. The volumes of interior atoms in the RC are also similar to those of water-soluble proteins, indicating the same compact packing for both types of proteins. The electrostatic potential of the cofactors was calculated. The results show an asymmetry in the potential between the two possible pathways of electron transfer, with the A branch being preferred electrostatically.

Photosynthetic reaction centers from purple bacteria exhibit an approximate twofold symmetry axis, which relates both the cofactors and the L and M subunits. For the reaction center from Rhodobacter sphaeroides, deviations from this twofold symmetry axis have been quantitated by superposing, by a 180 degrees rotation, the cofactors of the B branch onto the A branch and the M subunit onto the L subunit. An alignment of the sequences of the L and M subunits from four purple bacteria, one green bacterium, and the D1 and D2 subunits of a photosystem II-containing green alga is presented. The residues that are conserved in all six species are shown in relation to the structure of Rb. sphaeroides and their possible role in the function of the reaction center is discussed. A method is presented for characterizing the exposure of alpha-helices to the membrane based on the periodicity of conserved residues. This method may prove useful for modeling the three-dimensional structures of membrane proteins.

The three-dimensional structures of the cofactors and protein subunits of the reaction center (RC) from the carotenoidless mutant strain of Rhodobacter sphaeroides R-26 and the wild-type strain 2.4.1 have been determined by x-ray diffraction to resolutions of 2.8 A and 3.0 A with R values of 24% and 26%, respectively. The bacteriochlorophyll dimer (D), bacteriochlorophyll monomers (B), and bacteriopheophytin monomers (phi) form two branches, A and B, that are approximately related by a twofold symmetry axis. The cofactors are located in hydrophobic environments formed by the L and M subunits. Differences in the cofactor-protein interactions between the A and B cofactors, as well as between the corresponding cofactors of Rb, sphaeroides and Rhodopseudomonas viridis [Michel, H., Epp, O. & Deisenhofer, J. (1986) EMBO J. 3, 2445-2451], are delineated. The roles of several structural features in the preferential electron transfer along the A branch are discussed. Two bound detergent molecules of beta-octyl glucoside have been located near BA and BB. The environment of the carotenoid, C, that is present in RCs from Rb. sphaeroides 2.4.1 consists largely of aromatic residues of the M subunit. A role of BB in the triplet energy transfer from D to C and the reason for the preferential ease of removal of BB from the RC is proposed.

The three-dimensional structure of the cofactors of the reaction center of Rhodobacter sphaeroides R-26 has been determined by x-ray diffraction and refined at a resolution of 2.8 A with an R value of 26%. The main features of the structure are similar to the ones determined for Rhodopseudomonas viridis [Michel, H., Epp, O. & Deisenhofer, J. (1986) EMBO J. 5, 2445-2451]. The cofactors are arranged along two branches, which are approximately related to each other by a 2-fold symmetry axis. The structure is well suited to produce light-induced charge separation across the membrane. Most of the structural features predicted from physical and biochemical measurements are confirmed by the x-ray structure.

The three-dimensional structure of the reaction center (RC) from Rhodobacter sphaeroides has been determined by x-ray diffraction to a resolution of 2.8 A with an R value of 24%. The interactions of the protein with the primary quinone, QA, secondary quinone, QB, and the nonheme iron are described and compared to those of RCs from Rhodopseudomonas viridis. Structural differences between the QA and QB environments that contribute to the function of the quinones (the electron transfer from QA- to QB and the charge recombination of QA-, QB- with the primary donor) are delineated. The protein residues that may be involved in the protonation of QB are identified. A pathway for the doubly reduced QB to dissociate from the RC is proposed. The interactions between QB and the residues that have been changed in herbicide-resistant mutants are described. The environment of the nonheme iron is compared to the environments of metal ions in other proteins.