Laud P R, Campbell J W
Department of Biochemistry and Cell Biology, Rice University, Houston, TX 77251.
J Mol Evol. 1994 Jul;39(1):93-100. doi: 10.1007/BF00178254.
Tissue-specific isozymes of glutamine synthetase are present in elasmobranchs. A larger isozyme occurs in tissues in which the enzyme is localized in mitochondria (liver, kidney) whereas a smaller form occurs in tissues in which it is cytosolic (brain, spleen, etc.). The nucleotide sequence of spiny dogfish shark (Squalus acanthias) liver glutamine synthetase mRNA, derived from its cDNA, shows there are two in-frame initiation codons (AUG) at the N-terminus which will account for the size differences between the two isozymes. Initiation at the up-stream and down-stream sites would yield peptides of 45,406 and 41,869 mol. wts. representing the precursor of the mitochondrial isozyme and the cytosolic isozyme, respectively. The additional N-terminal 29 amino acids present in the mitochondrial isozyme precursor contains two putative cleavage sites based on the Arg-X-(Phe,Ile,Leu) motif. The predicted two-step processing would remove 14 of the 29 N-terminal amino acids. These 14 amino acids can be predicted to form a very strong amphipathic mitochondrial targeting signal. Their removal would yield a mature peptide of 43,680 mol. wt. The calculated mol. wts. based on the derived amino acid sequence are therefore in good agreement with previous estimates of an approximately 1.5-2-kDa difference between the M(r)s of the mitochondrial and cytosolic isozymes. A model for the evolution of the mitochondrial targeting of glutamine synthetase in vertebrates is proposed.
谷氨酰胺合成酶的组织特异性同工酶存在于软骨鱼类中。较大的同工酶出现在酶定位于线粒体的组织（肝脏、肾脏）中，而较小的形式出现在酶位于胞质溶胶的组织（大脑、脾脏等）中。来自其cDNA的棘鲨（Squalus acanthias）肝脏谷氨酰胺合成酶mRNA的核苷酸序列显示，在N端有两个符合读框的起始密码子（AUG），这将解释两种同工酶之间的大小差异。在上游和下游位点起始将分别产生分子量为45,406和41,869的肽，分别代表线粒体同工酶和胞质同工酶的前体。线粒体同工酶前体中额外的N端29个氨基酸基于Arg-X-（Phe，Ile，Leu）基序包含两个假定的切割位点。预测两步加工将去除29个N端氨基酸中的14个。这些14个氨基酸预计会形成一个非常强的两亲性线粒体靶向信号。去除它们将产生分子量为43,680的成熟肽。因此，根据推导的氨基酸序列计算出的分子量与先前估计的线粒体和胞质同工酶的M（r）之间约1.5 - 2 kDa的差异非常吻合。提出了脊椎动物谷氨酰胺合成酶线粒体靶向进化的模型。
