MAGI-1 interacts with beta-catenin and is associated with cell-cell adhesion structures.

The family of membrane-associated guanylate kinases (MAGUK) comprises peripheral membrane proteins involved in the formation of specialized cell-cell junctions. MAGUK proteins possess a conserved domain composition, containing PDZ, guanylate kinase, and SH3 or WW domains. MAGI-1 is a recently identified member of the MAGUK protein family. Three splice variantsof MAGI-1 have been characterized to date, including MAGI-1a, -1b, and -1c. MAGI-1b is predominantly associated with the crude membrane fraction. Here we show that the fifth PDZ domain of MAGI-1b is essential for membrane localization. We have also identified beta-catenin as a potential ligand for this PDZ domain. MAGI-1b forms complexes with beta-catenin and E-cadherin during the formation of cell-cell junctions in MDCK cells. In agreement with this observation, a significant portion of a GFP fusion of MAGI-1b localizes to the basolateral membrane of polarized MDCK cells.

MAGI-1, a membrane-associated guanylate kinase with a unique arrangement of protein-protein interaction domains.

Membrane-associated guanylate kinase (MAGUK) proteins participate in the assembly of multiprotein complexes on the inner surface of the plasma membrane at regions of cell-cell contact. MAGUKs are characterized by three types of protein-protein interaction modules: the PDZ domain, the Src homology 3 (SH3) domain, and the guanylate kinase (GuK) domain. The arrangement of these domains is conserved in all previously known MAGUKs: either one or three PDZ domains in the NH2-terminal half, followed by the SH3 domain, followed by a COOH-terminal GuK domain. In this report, we describe the cDNA cloning and subcellular distribution of MAGI-1, a MAGUK with three unique structural features: 1) the GuK domain is at the NH2 terminus, 2) the SH3 domain is replaced by two WW domains, and 3) it contains five PDZ domains. MAGI-1 mRNA was detected in several adult mouse tissues. Sequence analysis of overlapping cDNAs revealed the existence of three splice variants that are predicted to encode MAGI-1 proteins with different COOH termini. The longest variant, MAGI-1c, contains three bipartite nuclear localization signals in its unique COOH-terminal sequence and was found predominantly in the nucleus of Madin-Darby canine kidney cells. A shorter form lacking these signals was found primarily in membrane and cytoplasmic fractions. This distribution, which is reminiscent of that seen for the tight junction protein ZO-1, suggests that MAGI-1 may participate in the transmission of regulatory signals from the cell surface to the nucleus.

Localization of epidermal growth factor-stimulated Ras/Raf-1 interaction to caveolae membrane.

An essential step in the epidermal growth factor (EGF)-dependent activation of MAP kinase is the recruitment of Raf-1 to the plasma membrane. Here we present evidence that caveolae are the membrane site where Raf-1 is recruited. Caveolae fractions prepared from normal Rat-1 cells grown in the absence of serum were highly enriched in both EGF receptors and Ras. Thirty seconds after EGF was added to these cells Raf-1 began to appear in caveolae but not in non-caveolae membrane fractions. The maximum concentration was reached at 3 min followed by a decline over the next 60 min. During this time EGF receptors disappeared from the caveolae fraction while the concentration of Ras remained constant. The Raf-1 in this fraction was able to phosphorylate MAP kinase kinase, whereas cytoplasmic Raf-1 in the same cell was inactive. Elevation of cellular cAMP blocked the recruitment of Raf-1 to caveolae. Overexpression of Ha-RasV12 caused the recruitment of Raf-1 to caveolae independently of EGF stimulation, and this was blocked by the farnesyltransferase inhibitor BZA-5B. Finally, prenylation appeared to be required for localization of Ras to caveolae.

Polylysine and CVIM sequences of K-RasB dictate specificity of prenylation and confer resistance to benzodiazepine peptidomimetic in vitro.

BZA-5B, a benzodiazepine peptidomimetic, inhibits CAAX farnesyltransferase (FTase) and blocks attachment of farnesyl groups to oncogenic and wild-type H-Ras in animal cells. This compound slows the growth of cells transformed with oncogenic H-Ras at concentrations that do not affect the growth of nontransformed cells. This finding suggested that nontransformed cells may produce a form of Ras whose prenylation is resistant to BZA-5B. In the current studies, we found that FTase had a 50-fold higher affinity for K-RasB than for H-Ras in vitro. Farnesylation of K-RasB was inhibited by BZA-2B, the active form of BZA-5B, but only at concentrations that were 8-fold higher than those that inhibited farnesylation of H-Ras. K-RasB, but not H-Ras, was also a substrate for CAAX geranylgeranyltransferase-1 (GGTase-1), and its affinity for the enzyme was equal to that of Rap1B, an authentic leucine-terminated substrate for GGTase-1. Inhibition of the geranylgeranylation of K-RasB occurred only at high concentrations of BZA-2B. All of these properties of K-RasB were traced to the combined effects of its COOH-terminal CVIM sequence and the adjacent polylysine sequence, neither of which is present in H-Ras. These studies provide a potential explanation for the resistance of nontransformed cells to growth inhibition by BZA-5B. Inasmuch as the majority of Ras-related human cancers contain oncogenic versions of K-RasB rather than H-Ras, the current data suggest that in vitro studies of FTase inhibitors with potential anti-cancer activity should use authentic K-RasB as a substrate.

Benzodiazepine peptidomimetic BZA-5B interrupts the MAP kinase activation pathway in H-Ras-transformed Rat-1 cells, but not in untransformed cells.

A benzodiazepine peptidomimetic, BZA-5B, inhibits farnesylation of H-Ras and normalizes the morphology of Rat-1 cells transformed with H-RasV12 at concentrations that do not affect the growth of untransformed Rat-1 cells. In the current experiments, we show that BZA-5B decreases the active forms of enzymes in the mitogen-activated protein (MAP) kinase signaling cascade, including Raf, MAP kinase kinase (MEK), and MAP kinase, in cells transformed with H-RasV12. BZA-5B had no effect on these enzymes in cells transformed with H-RasV12,L189, which is geranylgeranylated rather than farnesylated. In cells transformed with H-RasV12, BZA-5B reduced the activities of enzymes in the MAP kinase pathway at concentrations that only partially blocked farnesylation of H-RasV12, suggesting that nonfarnesylated H-RasV12 is a dominant inhibitor of the action of farnesylated H-RasV12 in the BZA-5B treated cells. In untransformed Rat-1 cells, BZA-5B did not inhibit MAP kinase activity nor did it prevent the acute activation triggered by epidermal growth factor, even though farnesylated endogenous H-Ras was no longer detectable. These data raise the possibility that untransformed cells contain a form of Ras (K-Ras or N-Ras) whose prenylation is not inhibited by BZA-5B, thus allowing them to resist the effects of BZA-5B.

PxF, a prenylated protein of peroxisomes.

CAAX farnesyltransferase attaches a farnesyl group to proteins that terminate in the sequence CAAX, where C is cysteine, A is an aliphatic amino acid, and X is typically methionine or serine. A limited number of substrates for the CAAX farnesyltransferase have been identified in cultured cells. These include p21ras proteins and the nuclear lamins A and B. We describe here the use of a CAAX farnesyltransferase inhibitor, together with a hamster cell line that exhibits efficient uptake of [3H]mevalonate, as a means of identifying novel farnesylated proteins. One candidate protein was purified and its attached prenyl group identified as farnesyl. The predicted amino acid sequence of this protein, deduced from a cloned cDNA, terminates with the tetrapeptide Cys-Leu-Ile-Met, which conforms to the consensus sequence for recognition by farnesyltransferase. This farnesylated protein, designated PxF, is localized to the outer surface of peroxisomes as determined by indirect immunofluorescence and electron microscopy.

Benzodiazepine peptidomimetics: potent inhibitors of Ras farnesylation in animal cells.

Oncogenic Ras proteins transform animal cells to a malignant phenotype only when modified by farnesyl residues attached to cysteines near their carboxyl termini. The farnesyltransferase that catalyzes this reaction recognizes tetrapeptides of the sequence CAAX, where C is cysteine, A is an aliphatic amino acid, and X is a carboxyl-terminal methionine or serine. Replacement of the two aliphatic residues with a benzodiazepine-based mimic of a peptide turn generated potent inhibitors of farnesyltransferase [50 percent inhibitory concentration (IC50) < 1 nM]. Unlike tetrapeptides, the benzodiazepine peptidomimetics enter cells and block attachment of farnesyl to Ras, nuclear lamins, and several other proteins. At micromolar concentrations, these inhibitors restored a normal growth pattern to Ras-transformed cells. The benzodiazepine peptidomimetics may be useful in the design of treatments for tumors in which oncogenic Ras proteins contribute to abnormal growth, such as that of the colon, lung, and pancreas.