Shooter E M
Department of Neurobiology, Stanford University School of Medicine, Stanford, California 94305-5125, USA.
Annu Rev Neurosci. 2001;24:601-29. doi: 10.1146/annurev.neuro.24.1.601.
Adult male mouse submaxillary glands served as the preferred starting material for the isolation of the nerve growth factor (NGF) proteins in most of the isolation studies done. Two types of NGF proteins were isolated from extracts of the gland, a high-molecular-weight 7S NGF complex and a low-molecular-weight protein variously called NGF, betaNGF, or 2.5S NGF. The latter, which mediated all known biological functions of NGF, were closely related forms of a basic NGF dimer in which the N and C termini of two monomers (chains) were modified by proteolytic enzymes to different extents with no effect on biological activity. The betaNGF dimer showed a novel protein structure in which the two chains interacted non-covalently over a wide surface. Correspondingly, the betaNGF dimer was found to be unusually stable and the form through which NGFs actions were mediated at physiological concentrations. The betaNGF dimer was one of three subunits in 7S NGF; the other two were the gamma subunit, an arginine esteropeptidase or kallikrein, and the alpha subunit, an inactive kallikrein. Two zinc ions were also present in the complex and contributed greatly to its stability. There was much debate about whether 7S NGF was a specific protein complex of interacting subunits and, if so, what functions it might play in the biology of NGF. Observations of the inhibition of the enzyme activity of the gamma subunit and of the biological activity of betaNGF in 7S NGF were important in determining that 7S NGF was a naturally occurring complex and the sole source of NGF in the gland extract or in saliva. Specific interactions between the active site of the gamma subunit and the C-terminal arginine residues of the NGF chains, confirmed in the three-dimensional structure of 7S NGF, suggested a role for the gamma subunit in pro-NGF processing during the assembly of 7S NGF. In spite of the detailed knowledge of 7S NGF structure, no information on the role of this complex in the neurobiology of NGF has emerged. With the exception of the submaxillary gland of an African rodent, no other source of NGF has been convincingly shown to synthesize the alpha and gamma subunits, and they may well be irrelevant to NGFs actions.
在大多数神经生长因子(NGF)分离研究中,成年雄性小鼠的颌下腺是分离NGF蛋白的首选起始材料。从该腺体提取物中分离出两种类型的NGF蛋白,一种是高分子量的7S NGF复合物,另一种是低分子量蛋白,有多种名称,如NGF、βNGF或2.5S NGF。后者介导了NGF的所有已知生物学功能,是一种碱性NGF二聚体的紧密相关形式,其中两个单体(链)的N端和C端被蛋白水解酶不同程度地修饰,但对生物学活性没有影响。βNGF二聚体呈现出一种新颖的蛋白质结构,其中两条链在较大的表面上非共价相互作用。相应地,βNGF二聚体被发现异常稳定,且是NGF在生理浓度下起作用的形式。βNGF二聚体是7S NGF的三个亚基之一;另外两个是γ亚基,一种精氨酸酯肽酶或激肽释放酶,以及α亚基,一种无活性的激肽释放酶。该复合物中还存在两个锌离子,对其稳定性有很大贡献。关于7S NGF是否是相互作用亚基的特异性蛋白质复合物,如果是,它在NGF生物学中可能发挥什么作用,存在很多争论。观察7S NGF中γ亚基的酶活性抑制和βNGF的生物学活性抑制,对于确定7S NGF是一种天然存在的复合物以及腺体提取物或唾液中NGF的唯一来源很重要。γ亚基的活性位点与NGF链的C端精氨酸残基之间的特异性相互作用,在7S NGF的三维结构中得到证实,这表明γ亚基在7S NGF组装过程中对前体NGF加工有作用。尽管对7S NGF结构有详细了解,但关于该复合物在NGF神经生物学中的作用尚无信息。除了一种非洲啮齿动物的颌下腺外,没有其他令人信服的证据表明NGF的其他来源能合成α亚基和γ亚基,它们很可能与NGF的作用无关。