Guanine nucleotide exchange factors (GEFs) activate Rho GTPases by accelerating their GDP/GTP exchange. Trio and its paralog Kalirin (Kalrn) are unique members of the Rho-GEFs that harbor three catalytic domains: two functional GEF domains and a serine/threonine kinase domain. The N-terminal GEF domain activates Rac1 and RhoG GTPases, while the C-terminal GEF domain acts specifically on RhoA. Trio and Kalrn have an evolutionary conserved function in morphogenetic processes including neuronal development. De novo mutations in TRIO have lately been identified in patients with intellectual disability, suggesting that this protein family plays an important role in development and disease.

Phylogenetic and domain analysis revealed that a Kalrn/Trio ancestor originated in Prebilateria and duplicated in Urbilateria to yield Kalrn and Trio. Only few taxa outside the vertebrates retained both of these highly conserved proteins. To obtain first insights into their redundant or distinct functions in a vertebrate model system, we show for the first time a detailed comparative analysis of trio and kalrn expression in Xenopus laevis development. The mRNAs are maternally transcribed and expression increases starting with neurula stages. Trio and kalrn are detected in mesoderm/somites and different neuronal populations in the neural plate/tube and later also in the brain. However, only trio is expressed in migrating neural crest cells, while kalrn expression is detected in the cranial nerves, suggesting distinct functions. Thus, our expression analysis provides a good basis for further functional studies.

鸟嘌呤核苷酸交换因子（GEF）通过加速Rho GTPases的GDP / GTP交换来激活它们。Trio及其旁系同源蛋白Kalirin（Kalrn）是Rho-GEF的独特成员，其具有三个催化结构域：两个功能性GEF结构域和一个丝氨酸/苏氨酸激酶结构域。N末端的GEF结构域激活Rac1和RhoG GTPases，而C末端的GEF结构域专门作用于RhoA。Trio和Kalrn在包括神经元发育在内的形态发生过程中具有进化保守功能。最近在智障患者中发现了TRIO的从头突变，这表明该蛋白家族在发育和疾病中起着重要作用。

系统发育和域分析表明，Kalrn / Trio祖先起源于Prebilateria，在Urbilateria中重复产生Kalrn和Trio。脊椎动物以外只有很少的分类单元保留了这两个高度保守的蛋白质。为了获得在脊椎动物模型系统第一见解其冗余或不同的功能，我们示出了用于第一次的详细比较分析三人和kalrn在表达非洲爪蟾的发展。母体转录mRNA，从神经元阶段开始表达增加。在中胚层/ somites中以及在神经板/管中以及后来在大脑中的不同神经元种群中都检测到了Trio和Kaln。但是，只有三重奏在迁移的神经c细胞中表达Kalrn，而在颅神经中检测到kalrn表达，表明其功能不同。因此，我们的表达分析为进一步的功能研究提供了良好的基础。