The IB kinase (IKK) complex, composed of two catalytic subunits (IKK␣ and IKK) and a regulatory subunit (IKK␥), is the key enzyme in activation of nuclear factor B (NF-B). To study the mechanism and structure of the complex, we wanted to recombinantly express IKK in a model organism that lacks IKK. For this purpose, we have recombinantly reconstituted all three subunits together in yeast and have found that it is biochemically similar to IKK isolated from human cells. We show that there is one regulatory subunit per kinase subunit. Thus, the core subunit composition of IKK␣⅐⅐␥ complex is ␣ 1  1 ␥ 2 , and the core subunit composition of IKK⅐␥ is Nuclear factor B (NF-B)1 comprises a family of dimeric transcription factors that regulate the expression of over 150 genes involved in immune, stress, and antiapoptotic processes (1-4). Under normal circumstances, NF-B is tightly regulated so as to prevent inappropriate inflammation while allowing a rapid response to infection or stress. In unstimulated cells, NF-B is found predominantly in the cytoplasm in a complex with IB proteins (a family of inhibitory subunits including IB␣, IB, IB␥, IB⑀, and Bcl-3), which sequester NF-B and prevent its migration to the nucleus (5, 6). Diverse stimuli, including cytokines, bacterial and viral products, oxidants, and mitogens, lead to phosphorylation of two regulatory serine residues on IBs, which targets it for polyubiquitination and proteolytic degradation. This frees NF-B to move to the nucleus, where it binds to and stimulates the transcription of target genes (7).This phosphorylation is catalyzed by a large kinase complex, IB kinase (IKK) (8 -10). IKK is composed of two homologous kinase subunits, IKK␣ and IKK (85 and 87 kDa, respectively) and a 52-kDa regulatory subunit IKK␥ (8, 10, 11), also called NEMO (NF-B essential modulator) (12). IKK␥ is required for activation of IKK in response to TNF and other stimuli (13). IKK␣ and IKK each contain an N-terminal protein kinase domain (containing a canonical mitogen-activated protein kinase kinase activation loop (9)), a leucine zipper, and a helixloop-helix motif toward the C terminus (10). The catalytic subunits are associated with each other via their leucine zippers (11), and the helix-loop-helix domains are required for full IKK activation (14,15). It has been suggested that intramolecular interaction of the helix-loop-helix with the kinase domain is involved in IKK activation (14, 15). Studies of recombinant IKK␣ and IKK in insect cells indicate that the catalytic subunits are capable of forming both homodimers and heterodimers (11).Despite the high degree of sequence similarity between IKK␣ and IKK (52% overall identity and 65% identity in the kinase domains (10)), the two proteins differ. Whereas IKK is essential for induction of NF-B by cytokines, IKK␣ is essential for limb development and skin differentiation (16 -18). Moreover, IKK homodimer has ϳ30-fold higher activity toward IB␣ than IKK␣ (19). Other homologs of IKK␣ and IKK have been isolated, including TBK...
Oligodendrocytes, the myelinating cells of the central nervous system (CNS), are generated during development through the proliferation and differentiation of a distinct progenitor population. Not all oligodendrocyte progenitors generated during development differentiate, however, and large numbers of oligodendrocyte progenitors are present in the adult CNS, particularly in white matter. These "adult progenitors" can be identified through expression of the NG2 proteoglycan. Adult oligodendrocyte progenitors are thought to develop from the original pool of progenitors and in vitro are capable of differentiating into oligodendrocytes. Why these cells fail to differentiate in the intact CNS is currently unclear. Here we show that contact with CNS myelin inhibits the maturation of immature oligodendrocyte progenitors. The inhibition of oligodendrocyte progenitor maturation is a characteristic of CNS myelin that is not shared by several other membrane preparations including adult and neonatal neural membrane fractions, PNS myelin, or liver. This inhibition is concentration dependent, is reversible, and appears not to be mediated by either myelin basic protein or basic fibroblast growth factor. Myelin-induced inhibition of oligodendrocyte progenitor maturation provides a mechanism to explain the generation of a residual pool of immature oligodendrocyte progenitors in the mature CNS.
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