Characterization of the interaction of the monomeric GTP-binding protein Rab3a with geranylgeranyl transferase II

Distinct role of Rab3A and Rab3B in secretory activity of rat melanotrophs

Members of the Rab3 (A-D) subfamily of small GTPases are believed to play a key role in regulated exocytosis. These proteins share approximately 80% identity at amino acid level. The question of whether isoforms of Rab3 are functionally redundant was the subject of this study. We used RT-PCR analysis, in situ hybridization histochemistry, and confocal microscope-based analysis of immunocytochemistry to show that rat melanotrophs contain about equal amounts of Rab3A and Rab3B transcripts as well as proteins. Therefore, these cells are a suitable model to study the subcellular distribution and the role of these paralogous isoforms in regulated exocytosis. Secretory activity of single cells was monitored with patch-clamp capacitance measurements, and the cytosol was dialyzed with a high-calcium-containing patch pipette solution. Preinjection of antisense oligodeoxyribonucleotides specific to Rab3A, but not to Rab3B, induced a specific blockage of calcium-dependent secretory responses, indicating an exclusive requirement for Rab3A in melanotroph cell-regulated secretion. Although the injection of purified Rab3B protein was ineffective, the injection of recombinant Rab3A proteins into rat melanotrophs revealed that regulated secretion was stimulated by a GTP-bound Rab3A with an intact COOH terminus and inhibited by Rab3AT36N, impaired in GTP binding. These results indicate that Rab3A, but not Rab3B, enhances secretory output from rat melanotrophs and that their function is not redundant.

Hedgehog signal transduction proteins: contacts of the Fused kinase and Ci transcription factor with the kinesin-related protein Costal2

BACKGROUND

Hedgehog signaling proteins play important roles in development by controlling growth and patterning in various animals including Drosophila and mammals. Hedgehog signaling triggers changes in responsive cells through a novel transduction mechanism that ultimately controls the transcription of specific target genes via the activity of zinc finger transcription factors of the Cubitus interruptus/GLI family. In flies, key Hedgehog signal transduction components have been identified including the kinesin-related protein Costal2, the serinethreonine kinase Fused, and the PEST-containing protein Suppressor of Fused. These proteins control Cubitus interruptus cleavage, nucleo-cytoplasmic localization and activation. In fly embryos, Costal2, Fused, Suppressor of Fused and Cubitus interruptus are associated in at least one cytoplasmic complex, which interacts with the microtubules in a Hedgehog-dependent manner.

RESULTS

Here we identified and mapped direct interactions between Cos2, Fu, and Ci using an in vitro affinity assay and the yeast two-hybrid system.

CONCLUSIONS

Our results provide new insights into the possible mechanism of the cytosolic steps of Hedgehog transduction.

Interaction of farnesylated PRL-2, a protein-tyrosine phosphatase, with the beta-subunit of geranylgeranyltransferase II

Protein of regenerating liver (PRL)-1, -2, and -3 comprise a subgroup of closely related protein-tyrosine phosphatases featuring a C-terminal prenylation motif conforming to either the consensus sequence for farnesylation, CAAX, or geranylgeranylation, CCXX. Yeast two-hybrid screening for PRL-2-interacting proteins identified the beta-subunit of Rab geranylgeranyltransferase II (betaGGT II). The specific interaction of betaGGT II with PRL-2 but not with PRL-1 or -3 occurred in yeast and HeLa cells. Chimeric PRL-1/-2 molecules were tested for their interaction with betaGGT II, and revealed that the C-terminal region of PRL-2 is required for interaction, possibly the PRL variable region immediately preceeding the CAAX box. Additionally, PRL-2 prenylation is prequisite for betaGGT II binding. As prenylated PRL-2 is localized to the early endosome, we propose that this is where the interaction occurs. PRL-2 is not a substrate for betaGGT II, as isoprenoid analysis showed that PRL-2 was solely farnesylated in vivo. Co-expression of the alpha-subunit (alpha) of GGT II, betaGGT II, and PRL-2 resulted in alpha/betaGGT II heterodimer formation and prevented PRL-2 binding. Expression of PRL-2 alone inhibited the endogenous alpha/betaGGT II activity in HeLa cells. Together, these results indicate that the binding of alphaGGT II and PRL-2 to betaGGT II is mutually exclusive, and suggest that PRL-2 may function as a regulator of GGT II activity.

Biochemical characterization of Rab3-GTPase-activating protein reveals a mechanism similar to that of Ras-GAP

Small G proteins of the Rab family are regulators of intracellular vesicle traffic. Their intrinsic rate of GTP hydrolysis is very low but is enhanced by specific GTPase-activating proteins (GAPs) that switch G proteins to their inactive form. We have characterized the activity of recombinant Rab3-GAP on Rab3A in solution. The K(m) and K(d) values (75 microm) indicate a low affinity of Rab3-GAP for its substrate. The affinity is higher for the transition state analog Rab3A:GDP:AlF(x) (15 microm). The k(cat) (1 s(-)(1)) is within the range of values reported for other GAPs. A mutation in the switch I region of Rab3A disrupted the interaction with Rab3-GAP. Furthermore, Rabphilin, a putative target of Rab3, inhibited the activity of Rab3-GAP on Rab3. Therefore, the Rab3-GAP-binding site involves the switch I region of Rab3 and overlaps with the Rabphilin-binding domain. Substitution of a single arginine residue (Arg-728) of Rab3-GAP disrupted its catalytic activity but not its interaction with Rab3A. We propose that Rab3-GAP, like Ras- and Rho-GAPs, stabilizes the transition state of Rab3 and provides a critical arginine residue to accelerate the GTPase reaction.

Interaction cloning and characterization of the cDNA encoding the human prenylated rab acceptor (PRA1)

Rab proteins are small GTPases involved in the regulation of intracellular membrane traffic in mammalian cells. In order to find Rab-interacting proteins we performed a two-hybrid screening using a human brain cDNA library. Here we report the isolation of a full-length human cDNA clone coding for a protein of 185 amino acids. This protein interacts strongly with the Rab4b, Rab5a, and Rab5c proteins and weakly with Rab4a, Rab6, Rab7, Rab17, and Rab22 in the two-hybrid assay. Comparison with the Data Bank revealed that this clone represents the human homolog of the previously isolated rat Prenylated Rab Acceptor (rPRA1). Analysis of mRNA expression shows a single abundant mRNA of about 0.8 kb ubiquitously expressed. Western blot analysis of the overexpressed protein shows a band of the expected size equally distributed between cytosol and membranes.

The putative "switch 2" domain of the Ras-related GTPase, Rab1B, plays an essential role in the interaction with Rab escort protein

Posttranslational modification of Rab proteins by geranylgeranyltransferase type II requires that they first bind to Rab escort protein (REP). Following prenylation, REP is postulated to accompany the modified GTPase to its specific target membrane. REP binds preferentially to Rab proteins that are in the GDP state, but the specific structural domains involved in this interaction have not been defined. In p21 Ras, the alpha2 helix of the Switch 2 domain undergoes a major conformational change upon GTP hydrolysis. Therefore, we hypothesized that the corresponding region in Rab1B might play a key role in the interaction with REP. Introduction of amino acid substitutions (I73N, Y78D, and A81D) into the putative alpha2 helix of Myc-tagged Rab1B prevented prenylation of the recombinant protein in cell-free assays, whereas mutations in the alpha3 and alpha4 helices did not. Additionally, upon transient expression in transfected HEK-293 cells, the Myc-Rab1B alpha2 helix mutants were not efficiently prenylated as determined by incorporation of [3H]mevalonate. Metabolic labeling studies using [32P]orthophosphate indicated that the poor prenylation of the Rab1B alpha2 helix mutants was not directly correlated with major disruptions in guanine nucleotide binding or intrinsic GTPase activity. Finally, gel filtration analysis of cytosolic fractions from 293 cells that were coexpressing T7 epitope-tagged REP with various Myc-Rab1B constructs revealed that mutations in the alpha2 helix of Rab1B prevented the association of nascent (i.e., nonprenylated) Rab1B with REP. These data indicate that the Switch 2 domain of Rab1B is a key structural determinant for REP interaction and that nucleotide-dependent conformational changes in this region are largely responsible for the selective interaction of REP with the GDP-bound form of the Rab substrate.