Devineni N, Minamide L S, Niu M, Safer D, Verma R, Bamburg J R, Nachmias V T
Department of Cell and Developmental Biology, School of Medicine, University of Pennsylvania, Philadelphia, PA 19104-6058, USA.
Brain Res. 1999 Mar 27;823(1-2):129-40. doi: 10.1016/s0006-8993(99)01147-6.
The large G-actin pool in individual actively motile cells has been shown to be maintained primarily by the actin sequestering protein thymosin beta four (Tbeta4). It is not clear whether Tbeta4 or an isoform also plays a primary role in neural tissue containing highly motile axonal growth cones. To address this question we have made a definitive analysis of the relative contributions of all the known G-actin sequestering proteins: Tbeta4, Tbeta10, profilin, and phosphorylated (inactive) and unphosphorylated (potentially active) forms of both ADF and cofilin, in relation to the G-actin pool in developing chick brain at embryonic days 13 and 17. From our measurements we estimate the intracellular concentration of G-actin as 30-37 microM and of Tbeta4 as 50-60 microM in an 'average' brain cell in embryonic chick brain. No other beta thymosin isoforms were detected in these brain extracts. The ratio of soluble, unphosphorylated ADF to Tbeta4 is only 1:7 at 13 embryonic days, but increases to 1:4 at 17 days. Profilin and cofilin concentrations are an order of magnitude lower than Tbeta4. Combining the contributions of Tbeta4, unphosphorylated ADF and unphosphorylated cofilin, we estimate a mean G-actin critical concentration of approximately 0.45 microM and approximately 0.2 microM, respectively, in day 13 and day 17 embryonic brain extracts, suggesting a significant developmental decrease. We conclude that (a) Tbeta4 is the major actin sequestering protein in embryonic chick brain and the only beta thymosin isoform present; (b) ADF may play a significant developmental role, as its concentration changes significantly with age; (c) the known G-actin binding proteins can adequately account for the G-actin pool in embryonic chick brain.
在单个活跃运动的细胞中，大量的球状肌动蛋白（G-肌动蛋白）库主要由肌动蛋白隔离蛋白胸腺素β4（Tβ4）维持。目前尚不清楚Tβ4或其同种型在含有高度运动性轴突生长锥的神经组织中是否也起主要作用。为了解决这个问题，我们对所有已知的G-肌动蛋白隔离蛋白的相对贡献进行了明确分析：Tβ4、Tβ10、胸腺肌动蛋白结合蛋白（原肌球蛋白），以及肌动蛋白解聚因子（ADF）和丝切蛋白的磷酸化（无活性）和未磷酸化（潜在活性）形式，这些与胚胎第13天和第17天发育中的鸡脑内的G-肌动蛋白库有关。根据我们的测量，我们估计在胚胎期鸡脑的一个“普通”脑细胞中，G-肌动蛋白的细胞内浓度为30 - 37微摩尔，Tβ4的浓度为50 - 60微摩尔。在这些脑提取物中未检测到其他β-胸腺素同种型。在胚胎第13天，可溶性、未磷酸化的ADF与Tβ4 的比例仅为1:7，但在第
