Briggs M S, Gierasch L M
Biochemistry. 1984 Jul 3;23(14):3111-4. doi: 10.1021/bi00309a001.
Secretion of the Escherichia coli lambda receptor protein (LamB protein) appears from genetic evidence to be correlated with the predicted tendency of its signal sequence to adopt an alpha-helical conformation [Emr, S. D., & Silhavy, T. J. (1983) Proc. Natl. Acad. Sci. U. S. A. 80, 4599]. We have tested this hypothesis by synthesizing major portions of signal sequences from the wild-type and mutant LamB proteins and analyzing their conformations by circular dichroism. The wild-type signal sequence contains a seven-residue hydrophobic region flanked by a proline and a glycine. Chou-Fasman rules predict that this segment will adopt an alpha-helical conformation. An export-deficient mutant is missing four residues from this region; the helix-breaking glycine and proline are thus separated by only three residues, and an alpha helix is not predicted to form. In each of the export-restored revertants, either the glycine or the proline is replaced with a residue which promotes helix formation. The helix content of the synthetic signal sequence fragments on the basis of CD measurements supports the secondary structure hypothesis described above. The relative helicity in aqueous sodium dodecyl sulfate, lysolecithin, or trifluoroethanol is as follows: wild type greater than R2 (Pro----Leu) greater than R1 (Gly----Cys) much greater than deletion mutant.
从遗传学证据来看，大肠杆菌λ受体蛋白（LamB蛋白）的分泌似乎与其信号序列采用α螺旋构象的预测趋势相关[Emr, S. D., & Silhavy, T. J. (1983) 《美国国家科学院院刊》80, 4599]。我们通过合成野生型和突变型LamB蛋白信号序列的主要部分，并利用圆二色性分析它们的构象，来检验这一假设。野生型信号序列包含一个七残基的疏水区域，两侧分别是一个脯氨酸和一个甘氨酸。Chou-Fasman规则预测该片段将采用α螺旋构象。一个出口缺陷型突变体在该区域缺失了四个残基；因此，破坏螺旋的甘氨酸和脯氨酸仅被三个残基隔开，预计不会形成α螺旋。在每个恢复出口功能的回复突变体中，甘氨酸或脯氨酸被一个促进螺旋形成的残基所取代。基于圆二色性测量的合成信号序列片段的螺旋含量支持上述二级结构假设。在十二烷基硫酸钠水溶液、溶血卵磷脂或三氟乙醇中的相对螺旋度如下：野生型＞R2（脯氨酸→亮氨酸）＞R1（甘氨酸→半胱氨酸）＞＞缺失突变体。
