The t(10;11)(p13;q14) in the U937 cell line results in the fusion of the AF10 gene and CALM, encoding a new member of the AP-3 clathrin assembly protein family

The translocation t(10;11)(p13;q14) is a recurring chromosomal abnormality that has been observed in patients with acute lymphoblastic leukemia as well as acute myeloid leukemia. We have recently reported that the monocytic cell line U937 has a t(10;11)(p13;q14) translocation. Using a combination of positional cloning and candidate gene approach, we cloned the breakpoint and were able to show that AF10 is fused to a novel gene that we named CALM (Clathrin Assembly Lymphoid Myeloid leukemia gene) located at 11q14. AF10, a putative transcription factor, had recently been cloned as one of the fusion partners of MLL. CALM has a very high homology in its N-terminal third to the murine ap-3 gene which is one of the clathrin assembly proteins. The N-terminal region of ap-3 has been shown to bind to clathrin and to have a high-affinity binding site for phosphoinositols. The identification of the CALM/AF10 fusion gene in the widely used U937 cell line will contribute to our understanding of the malignant phenotype of this line.

Localization of platelet-derived growth factor-stimulated phosphorylation cascade to caveolae

Previously we showed that interleukin 1 beta stimulates the conversion of sphingomyelin to ceramide in the caveolae fraction of normal human fibroblasts. The ceramide, in turn, blocked platelet-derived growth factor (PDGF) stimulated DNA synthesis. We now present evidence that the PDGF receptor initiates signal transduction from caveolae. Cell fractionation and immunocytochemistry show caveolae to be the principal location of PDGF receptors at the cell surface. Multiple caveolae proteins acquire phosphotyrosine when PDGF binds to its receptor, but the hormone appears to have little effect on the tyrosine phosphorylation of non-caveolae membrane proteins. Five proteins known to interact with the phosphorylated receptor were found to be highly enriched in caveolae membrane. PDGF caused the concentration of three of these proteins to significantly increase in the caveolae fraction. Finally, PDGF stimulated the association of a 190-kDa phosphoprotein with the caveolae marker protein, caveolin. Therefore, ceramide may modulate PDGF receptor function directly in caveolae.

Co-purification and direct interaction of Ras with caveolin, an integral membrane protein of caveolae microdomains. Detergent-free purification of caveolae microdomains

Caveolae are plasma membrane specializations that have been implicated in signal transduction. Caveolin, a 21-24-kDa integral membrane protein, is a principal structural component of caveolae membranes in vivo. G protein alpha subunits are concentrated in purified preparations of caveolae membranes, and caveolin interacts directly with multiple G protein alpha subunits, including G(s), G(o), and G(i2). Mutational or pharmacologic activation of G alpha subunits prevents the interaction of caveolin with G proteins, indicating that inactive G alpha subunits preferentially interact with caveolin. Here, we show that caveolin interacts with another well characterized signal transducer, Ras. Using a detergent-free procedure for purification of caveolin-rich membrane domains and a polyhistidine tagged form of caveolin, we find that Ras and other classes of lipid-modified signaling molecules co-fractionate and co-elute with caveolin. The association of Ras with caveolin was further evaluated using two distinct in vitro binding assays. Wild-type H-Ras interacted with glutathione S-transferase (GST)-caveolin fusion proteins but not with GST alone. Using a battery of GST fusion proteins encoding distinct regions of caveolin, Ras binding activity was localized to a 41-amino acid membrane proximal region of the cytosolic N-terminal domain of caveolin. In addition, reconstituted caveolin-rich membranes (prepared with purified recombinant caveolin and purified lipids) interacted with a soluble form of wild-type H-Ras but failed to interact with mutationally activated soluble H-Ras (G12V). Thus, a single amino acid change (G12V) that constitutively activates Ras prevents or destabilizes this interaction. These results clearly indicate that (i) caveolin is sufficient to recruit soluble Ras onto lipid membranes and (ii) membrane-bound caveolin preferentially interacts with inactive Ras proteins. In direct support of these in vitro studies, we also show that recombinant overexpression of caveolin in intact cells is sufficient to functionally recruit a nonfarnesylated mutant of Ras (C186S) onto membranes, overcoming the normal requirement for lipid modification of Ras. Taken together, these observations suggest that caveolin may function as a scaffolding protein to localize or sequester certain caveolin-interacting proteins, such as wild-type Ras, within caveolin-rich microdomains of the plasma membrane.

Ligand-induced protease receptor translocation into caveolae: a mechanism for regulating cell surface proteolysis of the tissue factor-dependent coagulation pathway

The ability to regulate proteolytic functions is critical to cell biology. We describe events that regulate the initiation of the coagulation cascade on endothelial cell surfaces. The transmembrane protease receptor tissue factor (TF) triggers coagulation by forming an enzymatic complex with the serine protease factor VIIa (VIIa) that activates substrate factor X to the protease factor Xa (Xa). Feedback inhibition of the TF-VIIa enzymatic complex is achieved by the formation of a quaternary complex of TF-VIIa, Xa, and the Kunitz-type inhibitor tissue factor pathway inhibitor (TFPI). Concomitant with the downregulation of TF-VIIa function on endothelial cells, we demonstrate by immunogold EM that TF redistributes to caveolae. Consistently, TF translocates from the Triton X-100-soluble membrane fractions to low-density, detergent-insoluble microdomains that inefficiently support TF-VIIa proteolytic function. Downregulation of TF-VIIa function is dependent on quaternary complex formation with TFPI that is detected predominantly in detergent-insoluble microdomains. Partitioning of TFPI into low-density fractions results from the association of the inhibitor with glycosyl phosphatidylinositol anchored binding sites on external membranes. Free Xa is not efficiently bound by cell-associated TFPI; hence, we propose that the transient ternary complex of TF-VIIa with Xa supports translocation and assembly with TFPI in glycosphingolipid-rich microdomains. The redistribution of TF provides evidence for an assembly-dependent translocation of the inhibited TF initiation complex into caveolae, thus implicating caveolae in the regulation of cell surface proteolytic activity.

Phosphorylation of caveolin by src tyrosine kinases. The alpha-isoform of caveolin is selectively phosphorylated by v-Src in vivo

Caveolae are flask-shaped plasma membrane specializations that are thought to exist in most cell types. A 22-kDa protein, caveolin, is an integral membrane component of caveolae membranes in vivo. Previous studies have demonstrated that caveolin is phosphorylated on tyrosine by oncogenic viral Src (v-Src) and that caveolin is physically associated as a hetero-oligomeric complex with normal cellular Src (c-Src) and other Src family tyrosine kinases. Caveolin contains eight conserved tyrosine residues that may serve as potential substrates for Src. Here, we have begun to study the phosphorylation of caveolin by Src family tyrosine kinases both in vitro and in vivo. Using purified recombinant components, we first reconstituted the phosphorylation of caveolin by Src kinase in vitro. Microsequencing of Src-phosphorylated caveolin revealed that phosphorylation occurs within the extreme N-terminal region of full-length caveolin between residues 6 and 26. This region contains three tyrosine residues at positions 6, 14, and 25. Deletion mutagenesis demonstrates that caveolin residues 1-21 are sufficient to support this phosphorylation event, implicating tyrosine 6 and/or 14. In vitro phosphorylation of caveolin-derived synthetic peptides and site-directed mutagenesis directly show that tyrosine 14 is the principal substrate for Src kinase. In support of these observations, tyrosine 14 is the only tyrosine residue within caveolin that bears any resemblance to the known recognition motifs for Src family tyrosine kinases. In order to confirm or refute the relevance of these in vitro studies, we next analyzed the tyrosine phosphorylation of endogenous caveolin in v-Src transformed NIH 3T3 cells. In vivo, two isoforms of caveolin are known to exist: alpha-caveolin contains residues 1-178 and beta-caveolin contains residues 32-178. Only alpha-caveolin underwent tyrosine phosphorylation in v-Src transformed NIH 3T3 cells, although beta-caveolin is well expressed in these cells. As beta-caveolin lacks residues 1-31 (and therefore tyrosine 14), these in vivo studies directly demonstrate the validity of our in vitro studies. Because alpha- and beta-caveolin are known to assume a distinct but overlapping subcellular distribution within a single cell, v-Src phosphorylation of alpha-caveolin may only affect a subpopulation of caveolae that contain alpha-caveolin.

Molecular cloning of caveolin-3, a novel member of the caveolin gene family expressed predominantly in muscle

Caveolin, a 21-24-kDa integral membrane protein, is a principal component of caveolar membranes in vivo. Caveolin interacts directly with heterotrimeric G-proteins and can functionally regulate their activity. Recently, a second caveolin gene has been identified and termed caveolin-2. Here, we report the molecular cloning and expression of a third member of the caveolin gene gamily, caveolin-3. Caveolin-3 is most closely related to caveolin-1 based on protein sequence homology; caveolin-1 and caveolin-3 are approximately 65% identical and approximately 85% similar. A single stretch of eight amino acids (FED-VIAEP) is identical in caveolin-1, -2, and -3. This conserved region may represent a "caveolin signature sequence" that is characteristic of members of the caveolin gene family. Caveolin-3 mRNA is expressed predominantly in muscle tissue-types (skeletal muscle, diaphragm, and heart) and is selectively induced during the differentiation of skeletal C2C12 myoblasts in culture. In many respects, caveolin-3 is similar to caveolin-1: (i) caveolin-3 migrates in velocity gradients as a high molecular mass complex; (ii) caveolin-3 colocalizes with caveolin-1 by immunofluorescence microscopy and cell fractionation studies; and (iii) a caveolin-3-derived polypeptide functionally suppresses the basal GTPase activity of purified heterotrimeric G-proteins. Identification of a muscle-specific member of the caveolin gene family may have implications for understanding the role of caveolin in different muscle cell types (smooth, cardiac, and skeletal) as previous morphological studies have demonstrated that caveolae are abundant in these cells. Our results also suggest that other as yet unknown caveolin family members are likely to exist and may be expressed in a regulated or tissue-specific fashion.

Identification, sequence, and expression of caveolin-2 defines a caveolin gene family

Caveolin, a 21- to 24-kDa integral membrane protein, is a principal component of caveolae membranes. Caveolin interacts directly with heterotrimeric guanine nucleotide binding proteins (G proteins) and can functionally regulate their activity. Here, an approximately 20-kDa caveolin-related protein, caveolin-2, was identified through microsequencing of adipocyte-derived caveolin-enriched membranes; caveolin was retermed caveolin-1. Caveolins 1 and 2 are similar in most respects. mRNAs for both caveolin-1 and caveolin-2 are most abundantly expressed in white adipose tissue and are induced during adipocyte differentiation. Caveolin-2 colocalizes with caveolin-1, indicating that caveolin-2 also localizes to caveolae. However, caveolin-1 and caveolin-2 differ in their functional interactions with heterotrimeric G proteins, possibly explaining why caveolin-1 and -2 are coexpressed within a single cell.

Expression and characterization of recombinant caveolin. Purification by polyhistidine tagging and cholesterol-dependent incorporation into defined lipid membranes

Caveolin, a 22-24-kDa integral membrane protein, is a principal component of caveolar membranes in vivo. Caveolin has been proposed to function as a scaffolding protein to organize and concentrate signaling molecules within caveolae. Because of its unusual membrane topology, both the N- and C-terminal domains of caveolin remain entirely cytoplasmic and are not subject to luminal modifications that are accessible to other integral membrane proteins. Under certain conditions, caveolin also exists in a soluble form as a cytosolic protein in vivo. These properties make caveolin an attractive candidate for recombinant expression in Escherichia coli. Here, we successfully expressed recombinant full-length caveolin in E.coli. A polyhistidine tag was placed at its extreme C terminus for purification by Ni(2+)-nitrilotriacetic acid affinity chromatography. Specific antibody probes demonstrated that recombinant caveolin contained a complete N and C terminus. Recombinant caveolin remained soluble in solutions containing the detergent octyl glucoside and formed high molecular mass oligomers like endogenous caveolin. By electron microscopy, recombinant caveolin homo-oligomers appeared as individual spherical particles that were indistinguishable from endogenous caveolin homo-oligomers visualized by the same technique. As recombinant caveolin behaved as expected for endogenous caveolin, this provides an indication that recombinant caveolin can be used to dissect the structural and functional interaction of caveolin with other protein and lipid molecules in vitro. Recombinant caveolin was efficiently incorporated into lipid membranes as assessed by floatation in sucrose density gradients. This allowed us to use defined lipid components to assess the possible requirements for insertion of caveolin into membranes. Using a purified synthetic form of phosphatidylcholine (1,2-dioleoylphosphorylcholine), we observed that incorporation of caveolin into membranes was cholesterol-dependent; the addition of cholesterol dramatically increased the incorporation of caveolin into these phosphatidylcholine-based membranes by approximately 25-30-fold. This fits well with in vivo studies demonstrating that cholesterol plays an essential role in maintaining the structure and function of caveolae. Further functional analysis of these reconstituted caveolin-containing membranes showed that they were capable of recruiting a soluble recombinant form of G(i)2 alpha. This is in accordance with previous studies demonstrating that caveolin specifically interacts directly with multiple G protein alpha-subunits. Thus, recombinant caveolin incorporated into defined lipid membranes provides an experimental system in which the structure, function, and biogenesis of caveolin-rich membrane domains can be dissected in vitro.

M-caveolin, a muscle-specific caveolin-related protein

Caveolae, small invaginations of the plasma membrane, are a characteristic feature of many mammalian cells. The best-characterised caveolar protein is the integral membrane protein, VIP21-caveolin. We now describe a novel homologue of VIP21-caveolin, M-caveolin, which is expressed exclusively in muscle. M-caveolin was shown to be expressed in differentiated myotubes but not myoblasts. Epitope-tagged M-caveolin expressed in non-muscle cells was targetted to surface caveolae where it colocalized with endogenous VIP21-caveolin. M-caveolin may play a specialised role in the caveolae of muscle cells.

M-caveolin, a muscle-specific caveolin-related protein

Caveolae, small invaginations of the plasma membrane, are a characteristic feature of many mammalian cells. The best-characterised caveolar protein is the integral membrane protein, VIP21-caveolin. We now describe a novel homologue of VIP21-caveolin, M-caveolin, which is expressed exclusively in muscle. M-caveolin was shown to be expressed in differentiated myotubes but not myoblasts. Epitope-tagged M-caveolin expressed in non-muscle cells was targetted to surface caveolae where it colocalized with endogenous VIP21-caveolin. M-caveolin may play a specialised role in the caveolae of muscle cells.

Oligomeric structure of caveolin: implications for caveolae membrane organization

A 22-kDa protein, caveolin, is localized to the cytoplasmic surface of plasma membrane specializations called caveolae. We have proposed that caveolin may function as a scaffolding protein to organize and concentrate signaling molecules within caveolae. Here, we show that caveolin interacts with itself to form homooligomers. Electron microscopic visualization of these purified caveolin homooligomers demonstrates that they appear as individual spherical particles. By using recombinant expression of caveolin as a glutathione S-transferase fusion protein, we have defined a region of caveolin's cytoplasmic N-terminal domain that mediates these caveolin-caveolin interactions. We suggest that caveolin homooligomers may function to concentrate caveolin-interacting molecules within caveolae. In this regard, it may be useful to think of caveolin homooligomers as "fishing lures" with multiple "hooks" or attachment sites for caveolin-interacting molecules.

Clathrin binding and assembly activities of expressed domains of the synapse-specific clathrin assembly protein AP-3

We separately expressed the 58-kDa C-terminal, 42-kDa middle, 16-kDa C-terminal, and 33-kDa N-terminal regions of AP-3 (also called F1-20, AP180, NP185, and pp155), and determined their clathrin binding and assembly properties. The 58-kDa C-terminal region of AP-3 is able to bind to clathrin triskelia and assemble them into a homogeneous population of clathrin cages and will also bind to preassembled clathrin cages. The 42-kDa central region of AP-3 can bind to both clathrin triskelia and to clathrin cages, but cannot assemble clathrin triskelia into clathrin cages. The 16-kDa C-terminal region of AP-3 can bind to clathrin cages, but cannot bind to clathrin triskelia or assemble clathrin triskelia into clathrin cages. The clathrin binding activities of the 42-kDa central region and 16-kDa C-terminal region are weaker than the corresponding activity of either the 58-kDa C-terminal region or full-length AP-3. Previous efforts had mapped a clathrin binding site within the N-terminal 33 kDa of AP-3 (Murphy, J. E., Pleasure, I. T., Puszkin, S., Prasad, K., and Keen, J. H. (1991) J. Biol. Chem. 266, 4401-4408; Morris, S. A., Schroder, S., Plessmann, U., Weber, K., and Ungewickell, E. (1993) EMBO J. 12, 667-675). However, although the N-terminal 33 kDa of AP-3 is able to bind to clathrin triskelia (Murphy, J. E., Pleasure, I. T., Puszkin, S., Prasad, K., and Keen, J. H. (1991) J. Biol. Chem. 266, 4401-4408; Ye, W., and Lafer, E. M. (1995) J. Neurosci. Res. 41, 15-26), it does not promote their assembly into clathrin cages (Murphy, J. E., Pleasure, I. T., Puszkin, S., Prasad, K., and Keen, J. H. (1991) J. Biol. CHem. 266, 4401-4408; Ye, W., and Lafer, E. M. (1995) J. Neurosci. Res. 41, 15-26) or bind to preassembled clathrin cages (Ye, W., and Lafer, E. M. (1995) J. Neurosci. Res. 41, 15-26). It appears that the smallest functional unit that carries out all of the reported clathrin binding and assembly properties of AP-3, essentially as well as the full-length protein, is the 58-kDa C-terminal region.