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Cyclin cascades
HEADLINES
Sos activation
Aciculin in muscle junctions
ARONHEIM, A., ENGELBERG,D., LI, N., AL-ALAWI, N., SCHLESSINGER, I. and KARIN, M. (1994) Membrane targeting of the nucleotide exchange factor Sos is sufficient for activating the Ras signaling pathway Cell 78, 949-961
BELKIN, A. M. and BURRIDGE, K. (1994) Expression and localization of the phosphoglucomutase-related cytoskeletal protein, aciculin, in skeletal muscle }. Cell Sci. 107, 1993-2003
The pathway for relaying signals from growth factor receptors to the nucleus has become a familiar story. On binding ligand, receptors dimerize and autophosphorylate on tyrosine, allowing SH2-domain-containing effectors to be recruited. These include proteins with enzymatic activity, such as phospholipase C% as well as the adapter molecule Grb2. Grb2 recruits the Ras nuclentide-exchange factor Sos to the receptor complex (Sos binds m the SH3 domain of Grb2) and thereby activates it to stimulate GTP-GDP exchange on Ras; Ras then activates Raf and the MAP kinase cascade, ultimately influencing transcription factor function. Various mechanisms have been pro. posed to explain the regulation of the activities of signalling effectors, including Sos, by recruitment to the receptor complex; these include activation by phospho~ylatlon or by conformatlonal change as a result of binding, and localization of constitutively active effectors near their substrate molecules. This paper shows that Sos activates the Ras pathway without growth factor stimulation when it is er.gineered to carry membrane local. izatlon (mydstoylation, or farnesyl. ation and palmitoylation) signals, even when it lacks the Grb2-binding site. These findings strongly suggest that Sos is normally activated by Grb2 simply by its recruitment to the membrane. However, the authors also provide some evidence that activation may involve relief of an inhibitory effect of the C-terminal region. The mechanism of activation of Sos suggested by this study is remarkably similar to that reported for Raf earlier in the year: Rasactivates Raf by recruiting it to the membrane. In the case of Raf, however, components at the membrane are thought to then lock Raf in the kinase-active state; in contrast, Sos may be constitutively active, so membrane localization might serve simply to bring it close to its subs,rate.
Skeletal muscle cells form a wide variety of iunctional structures, such as the myotendinous junction (MTJ), the neuromuscular junction (NMj), and the Z-disc-membrane junctions hnown as costameres; these are important for the transmission of signals or mechanicP.I force. The muscle cell has thus proven a very useful model in which to study cytoskeletal linkages at contact structures. Belkin and Burridge describe the expression and localization of a newly identified cytoskeletal component, aciculin, in differentiating skeletal muscle cells. In culture, aciculin is absent from mouse myoblasts but starts to be expressed after the cells fuse into myotubes, and its level increases during subsequent differentiation and maturation. At first, it is present in focal contacts but later it becomes restricted to the Z-discs. In vivo, aciculin is present at the MTJ in developing avian skeletal muscle, and in the MTJ and costamere in mature skeletal muscle; it is not present at the NMJ. Aciculin is thus a new cytoskeletal component of skeletal muscle that is produced as myotubes form and is present at a subset of cell-matrix junctions. This points to an important function in the mechanical link between the cytoskeleton and the extracellular matrix in myotubes. Future studies should reveal what cell surface receptors aciculin interacts with and whether these interactions are direct or indirect. It will also be interesting to determine how aciculin is distributed in muscular disorders where the linkages between the cytoskeleton and the basement membrane are disturbed.
Cyclln cascades MAKEL~, T. P., TASSAN, I-P., NIGG, E. A., FRUTIGER, S., HUGHES, G. J. and WEINBERG, R. A. (1994) A cyclln associated with the CDK-acUvaUng klnase MO1 $ Nature 371,254-257 The eukaryotic cell cycle Is regulated by sequential activation of cyclln-dependent kinases (CDKs). Two distinct events are required for activation of these kinases: association of a cyclin subunit with the kinase catalytic subunit, and phosphorylarian of a conserved threonine residue in the kinase catalytic domain. Recent work has established that the MO15 protein kinase (also termed CAK for CDKactivating kinase), a distant relative of previously described CDKs, is responsible for this phosphorylation. In this paper, the authors have used a yeast two.hybrid system to search for cDNAs encoding proteins that interact with the human homologue of the MO15 protein kinase. They identified one clone encoding a product with low but significant sequence homology to known yeast and mammalian cyclins. This new cyclin, named cyclin H, associates with MO15 in viva: MO15 and a 34kDa polypeptide identified as cyclin H by protein sequencing could be coimmunoprecipitated. The association of cyclin H with MO15 functionally activates the kir ase as measured by its ability to phosphorylate and activate CDY2. These results indicate that MO15 is indeed a member of the CDK family. In addition, they show that a cyclin-CDK complex can function as a regulator of other cyclin-CDK complexes, suggesting that cyclin-CDK cascades exist in viva. Interestingly, the conserved threonine in CDKs that is phospho~ated by activated MO1S is also present in MO15. It is not yet clear whether phosphorylarian of this site is required for MO15 activation. If this proves to be the case, it will raise interesting questions about how the kinase brings about its own activation.
TRENDS IN CELL BIOLOGYVOL. 4 DECEMBER1994
This month's headlineswere contributed by Carolyn Bliss, Carol Featherstone, Donald Gullberg. Cad Smytheand StevenTheg, 419
Sos activation
Aciculin in muscle junctions
ARONHEIM, A., ENGELBERG,D., LI, N., AL-ALAWI, N., SCHLESSINGER, I. and KARIN, M. (1994) Membrane targeting of the nucleotide exchange factor Sos is sufficient for activating the Ras signaling pathway Cell 78, 949-961
BELKIN, A. M. and BURRIDGE, K. (1994) Expression and localization of the phosphoglucomutase-related cytoskeletal protein, aciculin, in skeletal muscle }. Cell Sci. 107, 1993-2003
The pathway for relaying signals from growth factor receptors to the nucleus has become a familiar story. On binding ligand, receptors dimerize and autophosphorylate on tyrosine, allowing SH2-domain-containing effectors to be recruited. These include proteins with enzymatic activity, such as phospholipase C% as well as the adapter molecule Grb2. Grb2 recruits the Ras nuclentide-exchange factor Sos to the receptor complex (Sos binds m the SH3 domain of Grb2) and thereby activates it to stimulate GTP-GDP exchange on Ras; Ras then activates Raf and the MAP kinase cascade, ultimately influencing transcription factor function. Various mechanisms have been pro. posed to explain the regulation of the activities of signalling effectors, including Sos, by recruitment to the receptor complex; these include activation by phospho~ylatlon or by conformatlonal change as a result of binding, and localization of constitutively active effectors near their substrate molecules. This paper shows that Sos activates the Ras pathway without growth factor stimulation when it is er.gineered to carry membrane local. izatlon (mydstoylation, or farnesyl. ation and palmitoylation) signals, even when it lacks the Grb2-binding site. These findings strongly suggest that Sos is normally activated by Grb2 simply by its recruitment to the membrane. However, the authors also provide some evidence that activation may involve relief of an inhibitory effect of the C-terminal region. The mechanism of activation of Sos suggested by this study is remarkably similar to that reported for Raf earlier in the year: Rasactivates Raf by recruiting it to the membrane. In the case of Raf, however, components at the membrane are thought to then lock Raf in the kinase-active state; in contrast, Sos may be constitutively active, so membrane localization might serve simply to bring it close to its subs,rate.
Skeletal muscle cells form a wide variety of iunctional structures, such as the myotendinous junction (MTJ), the neuromuscular junction (NMj), and the Z-disc-membrane junctions hnown as costameres; these are important for the transmission of signals or mechanicP.I force. The muscle cell has thus proven a very useful model in which to study cytoskeletal linkages at contact structures. Belkin and Burridge describe the expression and localization of a newly identified cytoskeletal component, aciculin, in differentiating skeletal muscle cells. In culture, aciculin is absent from mouse myoblasts but starts to be expressed after the cells fuse into myotubes, and its level increases during subsequent differentiation and maturation. At first, it is present in focal contacts but later it becomes restricted to the Z-discs. In vivo, aciculin is present at the MTJ in developing avian skeletal muscle, and in the MTJ and costamere in mature skeletal muscle; it is not present at the NMJ. Aciculin is thus a new cytoskeletal component of skeletal muscle that is produced as myotubes form and is present at a subset of cell-matrix junctions. This points to an important function in the mechanical link between the cytoskeleton and the extracellular matrix in myotubes. Future studies should reveal what cell surface receptors aciculin interacts with and whether these interactions are direct or indirect. It will also be interesting to determine how aciculin is distributed in muscular disorders where the linkages between the cytoskeleton and the basement membrane are disturbed.
Cyclln cascades MAKEL~, T. P., TASSAN, I-P., NIGG, E. A., FRUTIGER, S., HUGHES, G. J. and WEINBERG, R. A. (1994) A cyclln associated with the CDK-acUvaUng klnase MO1 $ Nature 371,254-257 The eukaryotic cell cycle Is regulated by sequential activation of cyclln-dependent kinases (CDKs). Two distinct events are required for activation of these kinases: association of a cyclin subunit with the kinase catalytic subunit, and phosphorylarian of a conserved threonine residue in the kinase catalytic domain. Recent work has established that the MO15 protein kinase (also termed CAK for CDKactivating kinase), a distant relative of previously described CDKs, is responsible for this phosphorylation. In this paper, the authors have used a yeast two.hybrid system to search for cDNAs encoding proteins that interact with the human homologue of the MO15 protein kinase. They identified one clone encoding a product with low but significant sequence homology to known yeast and mammalian cyclins. This new cyclin, named cyclin H, associates with MO15 in viva: MO15 and a 34kDa polypeptide identified as cyclin H by protein sequencing could be coimmunoprecipitated. The association of cyclin H with MO15 functionally activates the kir ase as measured by its ability to phosphorylate and activate CDY2. These results indicate that MO15 is indeed a member of the CDK family. In addition, they show that a cyclin-CDK complex can function as a regulator of other cyclin-CDK complexes, suggesting that cyclin-CDK cascades exist in viva. Interestingly, the conserved threonine in CDKs that is phospho~ated by activated MO1S is also present in MO15. It is not yet clear whether phosphorylarian of this site is required for MO15 activation. If this proves to be the case, it will raise interesting questions about how the kinase brings about its own activation.
TRENDS IN CELL BIOLOGYVOL. 4 DECEMBER1994
This month's headlineswere contributed by Carolyn Bliss, Carol Featherstone, Donald Gullberg. Cad Smytheand StevenTheg, 419