Jagtap Dhanashree D, Narahari Akkaladevi, Swamy Musti J, Mahale Smita D
Division of Structural Biology, National Institute for Research in Reproductive Health (Indian Council of Medical Research), Jehangir Merwanji Street, Parel, Mumbai-400012, India.
Biochim Biophys Acta. 2007 Jun;1774(6):723-31. doi: 10.1016/j.bbapap.2007.03.017. Epub 2007 Apr 5.
Prostate secretory protein of 94 amino acids (PSP94) is a small non-glycosylated, cysteine rich protein with a molecular mass of 10 kDa. It has also been referred to as beta-microseminoprotein (beta-MSP) and proteins homologous to it have been reported in a number of species. Comparison of the amino acid sequence of these proteins suggests that, it is a rapidly evolving protein. However, all the ten cysteine residues are well conserved in these homologues, indicating their possible role in maintaining the structure and function of these proteins. In the present study, PSP94 was purified from human seminal plasma and characterized further and it showed the presence of five disulfide bonds. Reduction of disulphide bonds of PSP94 led to significant changes in the secondary and tertiary structure of PSP94. CD of disulphide bond reduced PSP94 indicates an overall decrease in the beta sheet content from 79.8% to 46.4%. Tertiary structural changes as monitored by fluorescence quenching reveal that reduction of disulphide bonds of PSP94 followed by the modification of the free thiol groups leads to complete exposure of Trp32 and Trp92 and that one or more side chain carboxyl groups move closer to their indole side chains. Antibodies against native and modified PSP94 demonstrated that the changes following reduction of disulphide linkages are within the immunodominant region of the protein. Changes induced in the functional properties of PSP94, if any, by modification were investigated with respect to IgG binding as PSP94 has been reported to be similar to immunoglobulin binding factor purified from seminal plasma. A novel finding from this study is that both native PSP94 as well as modified protein have the ability to bind human IgG, suggesting the involvement of sequential epitopes of PSP94 in IgG binding.
94个氨基酸的前列腺分泌蛋白（PSP94）是一种分子量为10 kDa的小型非糖基化、富含半胱氨酸的蛋白质。它也被称为β-微精蛋白（β-MSP），并且在许多物种中都报道了与其同源的蛋白质。对这些蛋白质氨基酸序列的比较表明，它是一种快速进化的蛋白质。然而，所有10个半胱氨酸残基在这些同源物中都高度保守，表明它们可能在维持这些蛋白质的结构和功能中发挥作用。在本研究中，从人精浆中纯化了PSP94并进行了进一步表征，结果显示其存在五个二硫键。PSP94二硫键的还原导致其二级和三级结构发生显著变化。二硫键还原后的PSP94的圆二色性表明β折叠含量从79.8%总体下降到46.4%。通过荧光猝灭监测的三级结构变化表明，PSP94二硫键的还原以及随后游离巯基的修饰导致Trp32和Trp92完全暴露，并且一个或多个侧链羧基向其吲哚侧链靠近。针对天然和修饰后的PSP94的抗体表明，二硫键还原后的变化发生在该蛋白质的免疫显性区域内。由于据报道PSP94与从精浆中纯化的免疫球蛋白结合因子相似，因此针对IgG结合研究了修饰对PSP94功能特性的影响（如果有）。本研究的一个新发现是，天然PSP94以及修饰后的蛋白质都具有与人IgG结合的能力，这表明PSP94的连续表位参与了IgG结合。
