Law Richard, Harper Sandy, Speicher David W, Discher Dennis E
Department of Chemical and Biomolecular Engineering, School of Engineering and Applied Science, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6315, USA.
J Biol Chem. 2004 Apr 16;279(16):16410-6. doi: 10.1074/jbc.M313107200. Epub 2004 Feb 3.
Protein extensibility appears to be based broadly on conformational changes that can in principle be modulated by protein-protein interactions. Spectrin family proteins, with their extensible three-helix folds, enable evaluation of dimerization effects at the single molecule level by atomic force microscopy. Although some spectrin family members function physiologically only as homodimers (e.g. alpha-actinin) or are strictly monomers (e.g. dystrophin), alpha- and beta-spectrins are stable as monomeric forms but occur physiologically as alpha,beta-heterodimers bound laterally lengthwise. For short constructs of alpha- and beta-spectrin, either as monomers or as alpha,beta-dimers, sawtooth patterns in atomic force microscopy-forced extension show that unfolding stochastically extends repeats approximately 4-5-fold greater in length than native conformations. For both dimers and monomers, distributions of unfolding lengths appear bimodal; major unfolding peaks reflect single repeats, and minor unfolding peaks at twice the length reflect tandem repeats. Cooperative unfolding thus propagates through helical linkers between serial repeats (1, 2). With lateral heterodimers, however, the force distribution is broad and shifted to higher forces. The associated chains in a dimer can stay together and unfold simultaneously in addition to unfolding independently. Weak lateral interactions do not inhibit unfolding, but strong lateral interactions facilitate simultaneous unfolding analogous to serial repeat coupling within spectrin family proteins.
蛋白质的可伸展性似乎广泛基于构象变化，原则上这些构象变化可由蛋白质 - 蛋白质相互作用调节。血影蛋白家族蛋白质具有可伸展的三螺旋折叠结构，能够通过原子力显微镜在单分子水平评估二聚化效应。尽管一些血影蛋白家族成员在生理功能上仅作为同二聚体（如α - 辅肌动蛋白）或严格作为单体（如肌营养不良蛋白），但α - 和β - 血影蛋白以单体形式是稳定的，而在生理状态下以α,β - 异二聚体的形式纵向侧向结合。对于α - 和β - 血影蛋白的短构建体，无论是作为单体还是作为α,β - 二聚体，原子力显微镜强制伸展中的锯齿状模式表明，展开随机使重复序列延伸的长度比天然构象大约长4 - 5倍。对于二聚体和单体，展开长度的分布都呈现双峰；主要的展开峰反映单个重复序列，而长度为两倍的较小展开峰反映串联重复序列。因此，协同展开通过连续重复序列之间的螺旋连接体传播（1, 2）。然而，对于侧向异二聚体，力的分布较宽且向更高的力偏移。二聚体中的相关链除了独立展开外，还可以保持在一起并同时展开。弱的侧向相互作用不会抑制展开，但强的侧向相互作用促进同时展开，类似于血影蛋白家族蛋白质内的串联重复序列偶联。
