Department of Developmental Neurobiology, St. Jude Children's Research Hospital, Memphis, TN, USA.
Sci Rep. 2017 Jan 27;7:41428. doi: 10.1038/srep41428.
Proteins may undergo a type of posttranslational modification - polyglutamylation, where a glutamate residue is enzymatically linked to the γ-carboxyl group of a glutamate in the primary sequence of proteins and additional glutamates are then sequentially added via α-carboxyl-linkages to the growing glutamate side chain. Nna1 (a.k.a. CCP1) defines the 6-member cytosolic carboxypeptidase (CCP) family that metabolizes polyglutamate side chain and its loss results in neurodegeneration and male infertility. Whereas most CCPs catalyze hydrolysis of α-carboxyl-linked glutamates, CCP5 uniquely metabolizes the γ-carboxyl linked, branch point glutamate. Using purified recombinant mouse CCP5, we confirmed that it metabolized γ-carboxyl-linked glutamate of synthetic substrates and tubulin. Despite this unique feature and its indispensible functions in lower species, we found that unlike Nna1, CCP5 is not essential for neuronal survival in mouse. CCP5 deficiency does cause male infertility. However, the mechanism by which this occurs is distinct from that of Nna1 loss. Instead, it is phenotypically reminiscent of the infertility of olt mice. Our findings suggest that Nna1 and CCP5 do not work coordinately in the same pathway in either the nervous system or spermatogenesis. This is the first study addressing the function of CCP5 in mammals.
蛋白质可能会经历一种翻译后修饰——多聚谷氨酸化，其中谷氨酸残基通过酶促反应与蛋白质一级序列中的谷氨酸的γ-羧基连接，然后通过α-羧基连接将额外的谷氨酸依次添加到不断增长的谷氨酸侧链上。Nna1（又名 CCP1）定义了 6 成员胞质羧肽酶（CCP）家族，该家族代谢多聚谷氨酸侧链，其缺失会导致神经退行性变和男性不育。虽然大多数 CCP 催化α-羧基连接的谷氨酸的水解，但 CCP5 独特地代谢γ-羧基连接的分支点谷氨酸。使用纯化的重组小鼠 CCP5，我们证实它代谢了合成底物和微管蛋白的γ-羧基连接谷氨酸。尽管具有这种独特的特征及其在低等生物中的不可或缺的功能，但我们发现与 Nna1 不同，CCP5 不是小鼠神经元存活所必需的。CCP5 缺乏确实会导致男性不育。然而，这种情况发生的机制与 Nna1 缺失不同。相反，它在表型上类似于 olt 小鼠的不育症。我们的研究结果表明，Nna1 和 CCP5 在神经系统或精子发生中并不在同一途径中协同工作。这是首次研究 CCP5 在哺乳动物中的功能。
