Makorin 1 controls embryonic patterning by alleviating Bruno-mediated repression of oskar translation

Makorins are an evolutionary conserved family of proteins that contain C3H-type zinc finger modules and a RING E3 ubiquitin ligase domain. Previous analysis indicated a maternal role for Makorin 1 (Mkrn1) in Drosophila embryonic patterning and germ cell specification, but the underlying mechanism has remained elusive. Here, we show that Mkrn1 is specifically required for translational activation of oskar, which encodes a critical regulator of axis specification and germ plasm assembly. We demonstrate that Mkrn1 interacts with poly(A) binding protein (pAbp) and specifically binds osk 3’ UTR adjacent to A-rich sequences. The binding of Mkrn1 to osk 3’UTR occurs in a region that overlaps with Bruno responsive elements (BRE), previously shown to have a dual role in regulating osk translation. We observe an increased association of the translational repressor Bruno (Bru) with osk mRNA upon depletion of Mkrn1, implying that both proteins compete with each other for osk binding. Consistently, reducing Bru dosage is sufficient to partially rescue osk translation and the embryonic lethality associated with Mkrn1 alteration. Thus, we conclude that Mkrn1 controls embryonic patterning and germ cell formation by specifically activating osk translation via displacing Bru from its 3’ UTR

Makorin家族是一类进化保守的蛋白质家族，该家族成员均含有C3H型锌指结构域（C3H-type zinc finger modules）与RING型E3泛素连接酶结构域（RING E3 ubiquitin ligase domain）。既往研究显示，果蝇中的Makorin 1（Mkrn1）在胚胎模式形成与生殖细胞特化过程中发挥母体功能，但其潜在分子机制至今仍不明确。本研究发现，Mkrn1是oskar（osk）基因翻译激活的特异性必需因子，而oskar编码轴特化与生殖质组装的关键调控蛋白。我们证实，Mkrn1可与多聚腺苷酸结合蛋白（poly(A) binding protein, pAbp）相互作用，并特异性结合osk 3’非翻译区（3’ UTR）中富A序列的邻近区域。Mkrn1与osk 3’UTR的结合区域与布鲁诺应答元件（Bruno responsive elements, BRE）存在重叠，而BRE此前被证实对osk的翻译调控具有双重功能。研究观察到，在Mkrn1表达敲低后，翻译抑制因子布鲁诺（Bruno, Bru）与osk mRNA的结合水平显著升高，提示二者竞争性结合osk mRNA。与此一致的是，降低Bru的表达剂量足以部分挽救osk翻译缺陷以及Mkrn1异常所引发的胚胎致死表型。综上，本研究证实Mkrn1通过将布鲁诺蛋白从osk的3’ UTR上置换下来，特异性激活osk翻译，进而调控果蝇胚胎模式形成与生殖细胞生成。