Zisch A H, Stallcup W B, Chong L D, Dahlin-Huppe K, Voshol J, Schachner M, Pasquale E B
The Burnham Institute, La Jolla, California 92037, USA.
J Neurosci Res. 1997 Mar 15;47(6):655-65. doi: 10.1002/(sici)1097-4547(19970315)47:6<655::aid-jnr12>3.0.co;2-u.
The L1 family comprises transmembrane cell adhesion molecules of the immunoglobulin superfamily that play an important role in neuronal migration and axon outgrowth, fasciculation, and myelination. Consistent with a crucial role in developmental processes, mutations in L1 cause severe brain malformations. Although L1 activates intracellular signaling pathways, little is known about the membrane proximal events of L1 signaling. The cytoplasmic domains of L1 family proteins contain several conserved tyrosine residues that are potential targets for receptor tyrosine kinases. Here, we report that the L1 family protein Ng-CAM is phosphorylated on tyrosine in embryonic day 13 chicken retina. This is the first demonstration of in vivo tyrosine phosphorylation of an L1-like molecule. Because chicken embryo kinase 5 (Cek5) is a receptor tyrosine kinase expressed in neuronal processes and activated in the chicken embryonic retina, we have analyzed the possible role of Cek5 in L1 phosphorylation. The rat glioblastoma cell line B28 was stably transfected with human L1. Additional transient transfection with Cek5 cDNA led to expression of Cek5 in its tyrosine-phosphorylated, activated form. Biochemical analysis revealed that L1 is phosphorylated on tyrosine in Cek5-transfected cells but not in control transfectants. Furthermore, direct phosphorylation of the L1 cytoplasmic domain by Cek5 was demonstrated in an in vitro kinase assay. Tyrosine phosphorylation may represent a novel mechanism of signal cascade initiation through L1.
L1家族由免疫球蛋白超家族的跨膜细胞粘附分子组成，这些分子在神经元迁移、轴突生长、成束和髓鞘形成中发挥重要作用。与在发育过程中的关键作用一致，L1突变会导致严重的脑畸形。尽管L1激活细胞内信号通路，但对L1信号传导的膜近端事件知之甚少。L1家族蛋白的胞质结构域包含几个保守的酪氨酸残基，这些残基是受体酪氨酸激酶的潜在靶点。在此，我们报道L1家族蛋白Ng-CAM在鸡胚胎第13天视网膜中的酪氨酸被磷酸化。这是首次证明L1样分子在体内的酪氨酸磷酸化。由于鸡胚胎激酶5（Cek5）是一种在神经元突起中表达并在鸡胚胎视网膜中被激活的受体酪氨酸激酶，我们分析了Cek5在L1磷酸化中的可能作用。用人类L1稳定转染大鼠胶质母细胞瘤细胞系B28。用Cek5 cDNA进行额外的瞬时转染导致Cek5以其酪氨酸磷酸化的激活形式表达。生化分析表明，在转染Cek5的细胞中L1的酪氨酸被磷酸化，而在对照转染细胞中则未被磷酸化。此外，在体外激酶试验中证明了Cek5对L1胞质结构域的直接磷酸化。酪氨酸磷酸化可能代表通过L1启动信号级联反应的一种新机制。
