Molecular basis for semidominance of missense mutations in the XANTHA-H (42-kDa) subunit of magnesium chelatase
Molecular basis for semidominance of missense mutations in the XANTHA-H (42-kDa) subunit of magnesium chelatase
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DOI：
10.1073/pnas.96.4.1744
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发表时间：
1999-02-16
影响因子：
11.1
通讯作者：
Hansson, M
中科院分区：
文献类型：
--
作者：
Hansson, A;Kannangara, CG;Hansson, M
在细菌叶绿素或叶绿素的生物合成过程中，140，70和42 kDa的三个蛋白质亚基相互作用，将Mg 2+插入原卟啉IX中。大麦中的半显性Chlorina-125、-157和-161突变体在这一步骤中缺乏，并且在进食5-氨基乙酰丙酸后积累原卟啉IX。Chlorina-125、-157和-161与隐性xantha-h突变体等位，分别含有G559 A、G806 A和C271 T突变。这些突变导致单个氨基酸取代的残基是保守的，在所有已知的一级结构的42-kDa亚基，在体外的互补和重建的镁螯合酶活性表明，42-kDa亚基是有缺陷的半显性Chlorina突变体。一种突变的蛋白质被保留在Chlorina质体中，不像在xantha-h质体中。杂合的Chlorina幼苗具有野生型幼苗的25-50%的Mg-螯合酶活性。在杂合的Chlorina幼苗中，活性和非活性的42-kDa亚基的共显性表达可能在强相互作用的42-kDa和70-kDa亚基之间产生两种类型的异二聚体。减少镁螯合酶活性的解释能力的异源二聚体组成的突变的42-kDa和野生型70-kDa的蛋白质结合到140-kDa的亚基。42-kDa亚基在ATP-ADP交换活性和与70-kDa亚基产生ATP酶活性的能力方面类似于再折叠变性多肽的分子伴侣。我们假设，协会的42 kDa的亚基与70 kDa的亚基，使他们能够形成一个特定的复合物与140 kDa的亚基，这种复合物插入Mg 2+到原卟啉IX。
During biosynthesis of bacteriochlorophyll or chlorophyll, three protein subunits of 140, 70, and 42 kDa interact to insert Mg2+ into protoporphyrin IX. The semidominant Chlorina-125, -157, and -161 mutants in barley are deficient in this step and accumulate protoporphyrin IX after feeding on 5-aminolevulinate. Chlorina-125, -157, and -161 are allelic to the recessive xantha-h mutants and contain G559A, G806A, and C271T mutations, respectively. These mutations cause single amino acid substitutions in residues that are conserved in all known primary structures of the 42-kDa subunit, In vitro complementation and reconstitution of Mg-chelatase activity show that the 42-kDa subunits are defective in the semidominant Chlorina mutants. A mutated protein is maintained in the Chlorina plastids, unlike in the xantha-h plastids. Heterozygous Chlorina seedlings have 25-50% of the the Mg-chelatase activity of wild-type seedlings, Codominant expression of active and inactive 42-kDa subunits in heterozygous Chlorina seedlings is likely to produce two types of heterodimers between the strongly interacting 42-kDa and 70-kDa subunits. Reduced Mg-chelatase activity is explained by the capacity of heterodimers consisting of mutated 42-kDa and wild-type 70-kDa protein to bind to the 140-kDa subunit. The 42-kDa subunit is similar to chaperones that refold denatured polypeptides with respect to its ATP-to-ADP exchange activity and its ability to generate ATPase activity with the 70-kDa subunit. We hypothesize that the association of the 42-kDa subunit with the 70-kDa subunit allows them to form a specific complex with the 140-kDa subunit and that this complex inserts Mg2+ into protoporphyrin IX.