United States Department of Agriculture, Agricultural Research Service, University of Illinois Urbana, IL, USA.
Front Plant Sci. 2012 Aug 8;3:175. doi: 10.3389/fpls.2012.00175. eCollection 2012.
In metazoans, receptor kinases control many essential processes related to growth and development and response to the environment. The receptor kinases in plants and animals are structurally similar but evolutionarily distinct and thus while most animal receptor kinases are tyrosine kinases the plant receptor kinases are classified as serine/threonine kinases. One of the best studied plant receptor kinases is Brassinosteroid Insensitive 1 (BRI1), which functions in brassinosteroid signaling. Consistent with its classification, BRI1 was shown in early studies to autophosphorylate in vitro exclusively on serine and threonine residues and subsequently numerous specific phosphoserine and phosphothreonine sites were identified. However, several sites of tyrosine autophosphorylation have recently been identified establishing that BRI1 is a dual-specificity kinase. This raises the paradox that BRI1 contains phosphotyrosine but was only observed to autophosphorylate on serine and threonine sites. In the present study, we demonstrate that autophosphorylation on threonine and tyrosine (and presumably serine) residues is a post-translational modification, ruling out a co-translational mechanism that could explain the paradox. Moreover, we show that in general, autophosphorylation of the recombinant protein appears to be hierarchical and proceeds in the order: phosphoserine > phosphothreonine > phosphotyrosine. This may explain why tyrosine autophosphorylation was not observed in some studies. Finally, we also show that the juxtamembrane domain of BRI1 is an activator of the kinase domain, and that kinase specificity (serine/threonine versus tyrosine) can be affected by residues outside of the kinase domain. This may have implications for identification of signature motifs that distinguish serine/threonine kinases from dual-specificity kinases.
在后生动物中，受体激酶控制着许多与生长和发育以及对环境的响应相关的基本过程。植物和动物中的受体激酶在结构上相似，但在进化上是不同的，因此，虽然大多数动物受体激酶是酪氨酸激酶，但植物受体激酶被归类为丝氨酸/苏氨酸激酶。研究最深入的植物受体激酶之一是油菜素内酯不敏感 1（BRI1），它在油菜素内酯信号转导中发挥作用。与它的分类一致，早期研究表明 BRI1 在体外仅在丝氨酸和苏氨酸残基上进行自身磷酸化，随后鉴定出许多特定的磷酸丝氨酸和磷酸苏氨酸位点。然而，最近发现了几个酪氨酸自身磷酸化位点，这表明 BRI1 是一种双特异性激酶。这就产生了一个悖论，即 BRI1 含有磷酸酪氨酸，但仅观察到在丝氨酸和苏氨酸位点上进行自身磷酸化。在本研究中，我们证明了在苏氨酸和酪氨酸（以及可能的丝氨酸）残基上的自身磷酸化是一种翻译后修饰，排除了一种可以解释这种悖论的共翻译机制。此外，我们还表明，一般来说，重组蛋白的自身磷酸化似乎是一个分级过程，顺序为：磷酸丝氨酸＞磷酸苏氨酸＞磷酸酪氨酸。这可能解释了为什么在某些研究中没有观察到酪氨酸自身磷酸化。最后，我们还表明，BRI1 的跨膜区是激酶结构域的激活剂，并且激酶特异性（丝氨酸/苏氨酸与酪氨酸）可以受到激酶结构域外的残基的影响。这可能对识别区分丝氨酸/苏氨酸激酶和双特异性激酶的特征基序具有重要意义。
