Molecular mechanisms of polymorphic CYP3A7 expression in adult human liver and intestine

Human CYP3A enzymes play a pivotal role in the metabolism of many drugs, and the variability of their expression among individuals may have a strong impact on the efficacy of drug treatment. However, the individual contributions of the four CYP3A genes to total CYP3A activity remain unclear. To elucidate the role of CYP3A7, we have studied its expression in human liver and intestine. In both organs, expression of CYP3A7 mRNA was polymorphic. The recently identified CYP3A7*1C allele was a consistent marker of increased CYP3A7 expression both in liver and intestine, whereas the CYP3A7*1B allele was associated with increased CYP3A7 expression only in liver. Because of the replacement of part of the CYP3A7 promoter by the corresponding region of CYP3A4, the CYP3A7*1C allele contains the proximal ER6 motif of CYP3A4. The pregnane X and constitutively activated receptors were shown to bind with higher affinity to CYP3A4-ER6 than to CYP3A7-ER6 motifs and transactivated only promoter constructs containing CYP3A4-ER6. Furthermore, we identified mutations in CYP3A7*1C in addition to the ER6 motif that were necessary only for activation by the constitutively activated receptor. We conclude that the presence of the ER6 motif of CYP3A4 mediates the high expression of CYP3A7 in subjects carrying CYP3A7*1C.     

The destination for single-pass membrane proteins is influenced markedly by the length of the hydrophobic domain

The tonoplast was proposed as a default destination of membrane-bound proteins without specific targeting signals. To investigate the nature of this targeting, we created type I fusion proteins with green fluorescent protein followed by the transmembrane domain of the human lysosomal protein LAMP1. We varied the length of the transmembrane domain from 23 to either 20 or 17 amino acids by deletion within the hydrophobic domain. The resulting chimeras, called TM23, TM20, and TM17, were expressed either transiently or stably in tobacco. TM23 clearly accumulated in the plasmalemma, as confirmed by immunoelectron microscopy. In contrast, TM17 clearly was retained in the endoplasmic reticulum, and TM20 accumulated in small mobile structures. The nature of the TM20-labeled compartments was investigated by coexpression with a marker localized mainly in the Golgi apparatus, AtERD2, fused to a yellow fluorescent protein. The strict colocalization of both fluorescent proteins indicated that TM20 accumulated in the Golgi apparatus. To further test the default destination of type I membrane proteins, green fluorescent protein was fused to the 19-amino acid transmembrane domain of the plant vacuolar sorting receptor BP-80. The resulting chimera also accumulated in the Golgi instead of in post-Golgi compartments, where native BP-80 localized. Additionally, when the transmembrane domain of BP-80 was lengthened to 22 amino acids, the reporter escaped the Golgi and accumulated in the plasma membrane. Thus, the tonoplast apparently is not a favored default destination for type I membrane proteins in plants. Moreover, the target membrane where the chimera concentrates is not unique and depends at least in part on the length of the membrane-spanning domain.     

Structure of the C-terminal FG-nucleoporin binding domain of Tap/NXF1

The vertebrate Tap protein is a member of the NXF family of shuttling transport receptors for nuclear export of mRNA. Tap has a modular structure, and its most C-terminal domain is important for binding to FG repeat-containing nuclear pore proteins (FG-nucleoporins) and is sufficient to mediate nuclear shuttling. We report the solution structure of this C-terminal domain, which is based on a distinctive arrangement of four alpha-helices and is joined to the next module by a flexible 12-residue Pro-rich linker. F617A Tap suppresses FG-nucleoporin binding by the most C-terminal domain that, together with the structure of the other modules from which Tap is constructed, provides a structural context for its nuclear shuttling function.     

Altered plastidic ATP/ADP-transporter activity influences potato ( Solanum tuberosum L.) tuber morphology, yield and composition of tuber starch

The metabolic function of the plastidic ATP/ADP transporter (AATP) in heterotrophic plastids was examined in transgenic potato plants that exhibited increased or decreased amounts of the protein. Altered mRNA levels correlated with activities of the plastidic ATP/ADP transporter. Potato tubers with decreased plastidic ATP/ADP transporter activities exhibited reduced starch contents whereas sense lines accumulated increased amounts of tuber starch. Starch from wild-type tubers had an amylose content of 18.8%, starch from antisense plants contained 11.5–18.0% amylose, whereas starch from sense plants had levels of 22.7–27.0%. The differences in physiological parameters were accompanied with altered tuber morphology. These changes are discussed with respect to the stromal ATP supply during starch biosynthesis.     

Role of the D-alanyl carrier protein in the biosynthesis of D-alanyl-lipoteichoic acid.

D-Alanyl-lipoteichoic acid (D-alanyl-LTA) is a widespread macroamphiphile which plays a vital role in the growth and development of gram-positive organisms. The biosynthesis of this polymer requires the enzymic activation of D-alanine for its transfer to the membrane-associated LTA (mLTA). A small, heat-stable, and acidic protein that is required for this transfer was purified to greater than 98% homogeneity from Lactobacillus casei ATCC 7469. This protein, previously named the D-alanine-membrane acceptor ligase (V. M. Reusch, Jr., and F. C. Neuhaus, J. Biol. Chem. 246:6136-6143, 1971), functions as the D-alanyl carrier protein (Dcp). The amino acid composition, beta-alanine content, and N-terminal sequence of this protein are similar to those of the acyl carrier proteins (ACPs) of fatty acid biosynthesis. The isolation of Dcp and its derivative, D-alanyl approximately Dcp, has allowed the characterization of two novel reactions in the pathway for D-alanyl-mLTA biosynthesis: (i) the ligation of Dcp with D-alanine and (ii) the transfer of D-alanine from D-alanyl approximately Dcp to a membrane acceptor. It has not been established whether the membrane acceptor is mLTA or another intermediate in the pathway for D-alanyl-mLTA biosynthesis. Since the D-alanine-activating enzyme (EC 6.1.1.13) catalyzes the ligation reaction, this enzyme functions as the D-alanine-Dcp ligase (Dcl). Dcl also ligated the ACPs from Escherichia coli, Vibrio harveyi, and Saccharopolyspora erythraea with D-alanine. In contrast to the relaxed specificity of Dcl in the ligation reaction, the transfer of D-alanine to the membrane acceptor was highly specific for Dcp and did not occur with other ACPs. This transfer was observed by using only D-[14C]alanyl approximately Dcp and purified L. casei membranes. Thus, D-alanyl approximately Dcp is an essential intermediate in the transfer of D-alanine from Dcl to the membrane acceptor. The formation of D-alanine esters of mLTA provides a mechanism for modulating the net anionic charge in the cell wall.     

Hormonal regulation of hepatic amino acid transport

The transport of 2-aminoisobutyric acid (AIB) into liver tissue was increased by both insulin and glucagon. We have now shown that these hormones do not stimulate the same transport system. Glucagon, possibly via cAMP, increased the hepatic uptake of AIB by a mechanism which resembled system A. This glucagon-sensitive system could be monitored by the use of the model amino acid MeAIB. In contrast, the insulin-stimulated system exhibited little or no affinity for MeAIB and will be referred to as system B. On the basis of other reports that the hepatic transport of AIB is almost entirely Na⁺ dependent and the present finding that the uptake of 2-aminobicyclo [2,2,1] heptane-2-carboxylic acid (BCH) was not stimulated by either hormone, we conclude that system B is Na⁺ dependent. Furthermore, insulin added to the perfusate of livers from glucagon-pretreated donors suppressed the increase in AIB or MeAIB uptake. Depending upon the specificities of systems A and B, both of which are unknown for liver tissue, the insulin/glucagon ratio may alter the composition of the intracellular pool of amino acids.