Role of cytoplasmic termini in sorting and shuttling of the aquaporin-2 water channel.

In mammals, the regulation of water homeostasis is mediated by the aquaporin-1 (AQP1) water channel, which localizes to the basolateral and apical membranes of the early nephron segment, and AQP2, which is translocated from intracellular vesicles to the apical membrane of collecting duct cells after vasopressin stimulation. Because a similar localization and regulation are observed in transfected Madin-Darby Canine Kidney (MDCK) cells, we investigated which segments of AQP2 are important for its routing to forskolin-sensitive vesicles and the apical membrane through analysis of AQP1-AQP2 chimeras. AQP1 with the entire COOH tail of AQP2 was constitutively localized in the apical membrane, whereas chimeras with shorter COOH tail segments of AQP2 were localized in the apical and basolateral membrane. AQP1 with the NH2 tail of AQP2 was constitutively localized in both plasma membranes, whereas AQP1 with the NH2 and COOH tail of AQP2 was sorted to intracellular vesicles and translocated to the apical membrane with forskolin. These data indicate that region N220-S229 is essential for localization of AQP2 in the apical membrane and that the NH2 and COOH tail of AQP2 are essential for trafficking of AQP2 to intracellular vesicles and its shuttling to and from the apical membrane.

Functionality of aquaporin-2 missense mutants in recessive nephrogenic diabetes insipidus.

Aquaporin-2 (AQP2) missense mutants in recessive nephrogenic diabetes insipidus (NDI) are all retained in the endoplasmic reticulum (ER), but some could function as water channels. No conclusions could be drawn about the water permeability (Pf) of others, because there was no method for quantifying AQP2 expression in the plasma membrane. We recently developed such a method, which has allowed us to study the functionality of these AQP2 mutants. Immunoblot analysis of membranes of injected oocytes revealed that all mutants (AQP2-G64R, AQP2-N68S, AQP2 T126M, AQP2-A147T, AQP2-R187C, AQP2-S216P) are expressed as unglycosylated and high-mannose glycosylated AQP2. The level of the high-mannose form of AQP2-A147T in the plasma membranes was low, indicating that this mutation has a less severe effect on proper folding. Analysis of Pf values and plasma membrane expression levels reveals that AQP2-N68S, AQP2-R187C and AQP2-S216P are non-functional, AQP2-A147T is as functional as wt-AQP2, while AQP2-T126M and AQP2-G64R retain 20% of the permeability of wt-AQP2. Since G64 is highly conserved between AQPs and expected to form essential interactions with other amino acids within AQP1, the residual functionality of AQP2-G64R is surprising. Our data furthermore indicate that an eventual therapy with chemical chaperones that restores the routing of AQP2 mutants to the apical membrane of collecting ducts cells might relieve NDI in patients encoding AQP2-A147T, and to a lesser extent AQP2-T126M and AQP2-G64R, but not in patients encoding AQP2-N68S, AQP2-R187C or AQP2-S216P.

Accumulation of rab4GTP in the cytoplasm and association with the peptidyl-prolyl isomerase pin1 during mitosis.

Transport through the endocytic pathway is inhibited during mitosis. The mechanism responsible for this inhibition is not understood. Rab4 might be one of the proteins involved as it regulates transport through early endosomes, is phosphorylated by p34(cdc2) kinase, and is translocated from early endosomes to the cytoplasm during mitosis. We investigated the perturbation of the rab4 GTPase cycle during mitosis. Newly synthesized rab4 was less efficiently targeted to membranes during mitosis. By subcellular fractionation of mitotic cells, we found a large increase of cytosolic rab4 in the active GTP-form, an increase not associated with the cytosolic rabGDP chaperone GDI. Instead, phosphorylated rab4 is in a complex with the peptidyl-prolyl isomerase Pin1 during mitosis, but not during interphase. Our results show that less efficient recruitment of rab4 to membranes and a bypass of the normal GDI-mediated retrieval of rab4GDP from early endosomes reduce the amount of rab4GTP on membranes during mitosis. We propose that phosphorylation of rab4 inhibits both the recruitment of rab4 effector proteins to early endosomes and the docking of rab4-containing transport vesicles. This mechanism might contribute to the inhibition of endocytic membrane transport during mitosis.

Rabaptin4, a novel effector of the small GTPase rab4a, is recruited to perinuclear recycling vesicles.

The small GTPase rab4a is associated with early endocytic compartments and regulates receptor recycling from early endosomes. To understand how rab4a mediates its function, we searched for proteins which associate with this GTPase and regulate its activity in endocytic transport. Here we identified rabaptin4, a novel effector molecule of rab4a. Rabaptin4 is homologous with rabaptin5 and contains a C-terminal deletion with respect to rabaptin5. Rabaptin4 preferentially interacts with rab4a-GTP and to a lesser extent with rab5aGTP. We identified a rab4a-binding domain in the N-terminal region of rabaptin4, and two binding sites for rab5, including a novel N-terminal rab5a-binding site. Rabaptin4 is a cytosolic protein that inhibits the intrinsic GTP hydrolysis rate of rab4a and is recruited by rab4a-GTP to recycling endosomes enriched in cellubrevin and internalized indocarbocyanine-3 (Cy3)-labelled transferrin. We propose that rabaptin4 assists in the docking of transport vesicles en route from early endosomes to recycling endosomes.

Isolation of a bovine full length cytochrome P450 (CYP3A) cDNA sequence and its functional expression in V79 cells.

From a bovine liver cDNA library in λMaxl a 1870 bp cDNA was isolated using the human CYP3A4 cDNA as a probe. The cDNA-deduced amino acid sequence encoded a protein of 507 amino acids and exhibited homologies of 76, 72 and 64% with canine CYP3A12, human CYP3A4 and rat CYP3A1, respectively. Furthermore, a very high homology of 91.7% was observed with the deduced amino acid sequence of a partial CYP3A cDNA from dwarf goat. A striking observation was that both the bovine and the goat cDNA exhibit a 4 amino acid extension at the C-terminus, which is due to a frame-shifting insertion of 2 nt. The bovine CYP3A cDNA was cloned in a retroviral vector, transfected to V79 cells and cells were selected for cytochrome P450 expression. The expressed enzyme was shown to catalyze the 6ß-hydroxylation of testosterone, which could also be observed in a V79 cell line expressing human CYP3A4. In the bovine CYP3A cell line, however, 6ß-hydroxytestosterone was not found to be the major metabolite. This cell line additionally showed high levels of hydroxylase activity at the 2ß and 12ß position of testosterone. The cDNA-expressed testosterone hydroxylase activity could be inhibited with the specific CYP3A inhibitors, tiamulin and ketoconazole.

A novel epitope tag for the detection of rabGTPases.

Rab GTPases are localized on the cytoplasmic surface of most intracellular organelles where they play a role in the regulation of vesicular transport. As it has been difficult to detect endogenous rab proteins by morphological methods, their localizations were often inferred from transfection experiments using epitope-tagged constructs. Because most of the available epitope tags are only recognitzed by mouse monoclonal antibodies they are often not suitable for double or triple label immunocytochemistry. To overcome this problem, we generated antibodies against a novel 10 amino acid X31 influenza hemagglutin epitope (NH). We here characterized these antibodies and document their utility for detecting early endosomal rab proteins N-terminally tagged with the NH decapeptide in morphological and biochemical assays.