The effects of donor age and anatomic site on cellular replication and differentiation were studied in adipocyte precursors cloned from epididymal and perirenal depots of young, middle-aged, and senescent rats. As animals aged from 3 to 29 mo, there was a progressive reduction in the proportion of cells capable of extensive replication in both depots. An inverse relation between clonal capacity for replication and differentiation was found. This relation was affected by donor site but not age. Aging was, however, associated with a reduction in the frequency of clones capable of full differentiation into cells with single, large, central lipid inclusions. Hence, age and donor site may affect adipocyte precursor replication and differentiation by different mechanisms.

We report crystal structures of the citrate and sn-glycerol-1-phosphate (G1P) complexes of (S)-3-O-geranylgeranylglyceryl phosphate synthase from Archaeoglobus fulgidus (AfGGGPS) at 1.55 and 2.0 Å resolution, respectively. AfGGGPS is an enzyme that performs the committed step in archaeal lipid biosynthesis, and it presents the first triose phosphate isomerase (TIM)-barrel structure with a prenyltransferase function. Our studies provide insight into the catalytic mechanism of AfGGGPS and demonstrate how it selects for the sn-G1P isomer. The replacement of "Helix 3" by a "strand" in AfGGGPS, a novel modification to the canonical TIMbarrel fold, suggests a model of enzyme adaptation that involves a "greasy slide" and a "swinging door." We propose functions for the homologous PcrB proteins, which are conserved in a subset of pathogenic bacteria, as either prenyltransferases or being involved in lipoteichoic acid biosynthesis. Sequence and structural comparisons lead us to postulate an early evolutionary history for AfGGGPS, which may highlight its role in the emergence of Archaea.The membrane lipids found in Archaea are a defining characteristic of this domain of life (1). These lipids are based on a core architecture where branched-chain saturated hydrocarbons are connected to glycerol through ether linkages (2, 3). In hyperthermophiles, two diphytanylglyceryl units are often linked covalently through their hydrocarbon tails to form tetraether lipids that completely span the membrane. In addition, archaeal membrane lipids have three general characteristics that distinguish them from their bacterial and eukaryotic counterparts (4). First, the phospholipid backbone is built upon the opposite glycerol stereoisomer, sn-glycerol-1-phosphate (G1P), 4 not the sn-glycerol-3-phosphate (G3P) backbone found in bacteria and eukaryotes. Second, the hydrophobic chains are isoprenoid derivatives instead of fatty acids. Third, the isoprenoid chains are bound to G1P through ether, not ester, linkages. Of these traits, the glycerol phosphate stereochemistry is the most distinctive because ether-linked lipids are known to exist in some eukaryotes and bacteria (5, 6), and phospholipid fatty acids have recently been described in Archaea (7). To date, however, there is no known exception to the G1P backbone stereochemistry of archaeal lipids or to the G3P backbone stereochemistry found in bacterial and eukaryotic lipids.The biosynthesis of archaeal membrane lipids is schematically illustrated in (Fig. 1). In brief, dimethylallyl diphosphate (DMAPP) and its isomer isopentenyl diphosphate are synthesized by a mevalonate-like pathway (2, 8). Long isoprenoid chains are produced from these fivecarbon precursors by consecutive condensations through the action of a prenyl diphosphate synthase. The committed step in archaeal lipid synthesis occurs with the formation of an ether linkage between G1P and an isoprenoid diphosphate, usually geranylgeranyl diphosphate (GGPP). Separate enzymes catalyze the sequential transfer of isoprenoid units onto ...

The cyanobacterial clock proteins KaiA and KaiB are proposed as regulators of the circadian rhythm in cyanobacteria. Mutations in both proteins have been reported to alter or abolish circadian rhythmicity. Here, we present molecular models of both KaiA and KaiB from the cyanobacteria Anabaena sp PCC7120 deduced by crystal structure analysis, and we discuss how clock-changing or abolishing mutations may cause their resulting circadian phenotype. The overall fold of the KaiA monomer is that of a four-helix bundle. KaiB, on the other hand, adopts an alpha-beta meander motif. Both proteins purify and crystallize as dimers. While the folds of the two proteins are clearly different, their size and some surface features of the physiologically relevant dimers are very similar. Notably, the functionally relevant residues Arg 69 of KaiA and Arg 23 of KaiB align well in space. The apparent structural similarities suggest that KaiA and KaiB may compete for a potential common binding site on KaiC.