Schreiber-Agus N, Chin L, Chen K, Torres R, Thomson C T, Sacchettini J C, DePinho R A
Department of Microbiology and Immunology, Albert Einstein College of Medicine, Bronx, New York 10461.
Oncogene. 1994 Nov;9(11):3167-77.
In mammals, current evidence supports the view that Myc-responsive activities are regulated in part through an intracellular balance between levels of transcriptionally-active Myc/Max heterodimers and those of transcriptionally-inert Max/Max, Mad/Max and Mxi1/Max complexes. To gain insight into the roles of Mad and Mxi1 in cellular growth and differentiation and to fortify key structure-function relationships from an evolutionary standpoint, low stringency hybridization screens were used to identify potential homologs of these Max-associated proteins in the zebra fish genome. A single class of cDNA clones that cross-hybridized both to human mad and mxi1 probes was shown to encode a putative protein with significantly greater homology to mammalian Mxi1 than to Mad, particularly in the basic and helix-loop-helix (bHLH) regions. The high degree of structural relatedness between vertebrate Mxi1 proteins apparent in molecular modelling studies was consistent with the findings that the HLH/leucine zipper (LZ) region of zMxi1 exhibited the same profile of dimerization specificities as its mammalian counterpart in the two-hybrid system and that zmxi1 could, like human mxi1 (Lahoz et al., 1994), suppress the oncogenic potential of mouse c-myc in a mammalian cell. Finally, a comparison of steady-state zc-myc and zmxi1 mRNA levels during zebra fish embryogenesis demonstrated (i) high levels of zc-myc relative to zmxi1 mRNA during initiation of organogenesis, a period characterized by intense growth and active differentiation and (ii) rising levels of zmxi1 mRNA during progression towards the terminally differentiated state. These contrasting patterns of developmental expression together with the capacity of zmxi1 to repress myc-induced transformation support a model for the regulation, by Max-associated proteins, of Myc functions in the control of normal cell development and neoplastic growth.
在哺乳动物中，目前的证据支持这样一种观点，即Myc反应活性部分是通过转录活性的Myc/Max异二聚体水平与转录惰性的Max/Max、Mad/Max和Mxi1/Max复合物水平之间的细胞内平衡来调节的。为了深入了解Mad和Mxi1在细胞生长和分化中的作用，并从进化的角度强化关键的结构-功能关系，采用低严谨度杂交筛选来鉴定斑马鱼基因组中这些与Max相关蛋白的潜在同源物。一类与人类mad和mxi1探针都能交叉杂交的cDNA克隆被证明编码一种推定蛋白，该蛋白与哺乳动物Mxi1的同源性显著高于与Mad的同源性，尤其是在碱性和螺旋-环-螺旋（bHLH）区域。分子建模研究中明显显示的脊椎动物Mxi1蛋白之间的高度结构相关性与以下发现一致：zMxi1的HLH/亮氨酸拉链（LZ）区域在双杂交系统中表现出与其哺乳动物对应物相同的二聚化特异性模式，并且zmxi1能够像人类mxi1（Lahoz等人，1994年）一样在哺乳动物细胞中抑制小鼠c-myc的致癌潜力。最后，对斑马鱼胚胎发育过程中稳态zc-myc和zmxi1 mRNA水平的比较表明：（i）在器官发生开始期间，相对于zmxi1 mRNA，zc-myc水平较高，这一时期的特征是强烈生长和活跃分化；（ii）在向终末分化状态进展过程中，zmxi1 mRNA水平上升。这些发育表达的对比模式以及zmxi1抑制myc诱导的转化的能力支持了一种模型，即由与Max相关的蛋白对Myc功能进行调节，以控制正常细胞发育和肿瘤生长。
