Institute of Molecular Biosciences, Massey University, Palmerston North, New Zealand.
PLoS One. 2012;7(4):e35163. doi: 10.1371/journal.pone.0035163. Epub 2012 Apr 4.
Membrane protein sorting is mediated by interactions between proteins and lipids. One mechanism that contributes to sorting involves patches of lipids, termed lipid rafts, which are different from their surroundings in lipid and protein composition. Although the nerve growth factor (NGF) receptors, TrkA and p75(NTR) collaborate with each other at the plasma membrane to bind NGF, these two receptors are endocytosed separately and activate different cellular responses. We hypothesized that receptor localization in membrane rafts may play a role in endocytic sorting. TrkA and p75(NTR) both reside in detergent-resistant membranes (DRMs), yet they responded differently to a variety of conditions. The ganglioside, GM1, caused increased association of NGF, TrkA, and microtubules with DRMs, but a decrease in p75(NTR). When microtubules were induced to polymerize and attach to DRMs by in vitro reactions, TrkA, but not p75(NTR), was bound to microtubules in DRMs and in a detergent-resistant endosomal fraction. NGF enhanced the interaction between TrkA and microtubules in DRMs, yet tyrosine phosphorylated TrkA was entirely absent in DRMs under conditions where activated TrkA was detected in detergent-sensitive membranes and endosomes. These data indicate that TrkA and p75(NTR) partition into membrane rafts by different mechanisms, and that the fraction of TrkA that associates with DRMs is internalized but does not directly form signaling endosomes. Rather, by attracting microtubules to lipid rafts, TrkA may mediate other processes such as axon guidance.
膜蛋白分选是由蛋白质和脂质之间的相互作用介导的。一种有助于分选的机制涉及到被称为脂筏的脂质斑块，它们在脂质和蛋白质组成上与周围环境不同。尽管神经生长因子（NGF）受体 TrkA 和 p75（NTR）在质膜上协同作用以结合 NGF，但这两种受体是分别内吞的，并激活不同的细胞反应。我们假设受体在膜筏中的定位可能在胞吞分拣中起作用。TrkA 和 p75（NTR）都存在于去污剂抗性膜（DRM）中，但它们对各种条件的反应不同。神经节苷脂 GM1 导致 NGF、TrkA 和微管与 DRM 的结合增加，但 p75（NTR）的结合减少。当微管通过体外反应诱导聚合并附着到 DRM 时，TrkA 而不是 p75（NTR）与 DRM 中的微管结合，并且与去污剂抗性内体部分结合。NGF 增强了 TrkA 和微管在 DRM 中的相互作用，但在激活的 TrkA 在去污剂敏感的膜和内体中被检测到的条件下，酪氨酸磷酸化的 TrkA 在 DRM 中完全不存在。这些数据表明，TrkA 和 p75（NTR）通过不同的机制分配到膜筏中，并且与 DRM 相关联的 TrkA 部分被内化，但不会直接形成信号内体。相反，通过将微管吸引到脂筏中，TrkA 可能介导其他过程，如轴突导向。
