Department of Cell Biology, Yale University School of Medicine, New Haven, CT 06510, USA.
J Cell Biol. 2011 Apr 4;193(1):219-33. doi: 10.1083/jcb.201009001. Epub 2011 Mar 28.
The membrane of the primary cilium is continuous with the plasma membrane but compositionally distinct. Although some membrane proteins concentrate in the cilium, others such as podocalyxin/gp135 are excluded. We found that exclusion reflects a saturable selective retention mechanism. Podocalyxin is immobilized by its PDZ interaction motif binding to NHERF1 and thereby to the apical actin network via ERM family members. The retention signal was dominant, autonomous, and transferable to membrane proteins not normally excluded from the cilium. The NHERF1-binding domains of cystic fibrosis transmembrane conductance regulator and Csk-binding protein were also found to act as transferable retention signals. Addition of a retention signal could inhibit the ciliary localization of proteins (e.g., Smoothened) containing signals that normally facilitate concentration in the ciliary membrane. Proteins without a retention signal (e.g., green fluorescent protein-glycosylphosphatidylinositol) were found in the cilium, suggesting entry was not impeded by a diffusion barrier or lipid microdomain. Thus, a hierarchy of interactions controls the composition of the ciliary membrane, including selective retention, selective inclusion, and passive diffusion.
初级纤毛的膜与质膜连续，但组成上不同。虽然一些膜蛋白集中在纤毛中，但其他蛋白如足细胞蛋白聚糖/gp135 则被排除在外。我们发现，这种排除反映了一种可饱和的选择性保留机制。足细胞蛋白聚糖通过其 PDZ 相互作用基序与 NHERF1 结合，从而通过 ERM 家族成员与顶端肌动蛋白网络结合而被固定。保留信号是显性的、自主的，并且可以转移到通常不被排除在纤毛之外的膜蛋白上。囊性纤维化跨膜电导调节体和 Csk 结合蛋白的 NHERF1 结合结构域也被发现可以作为可转移的保留信号。添加保留信号可以抑制含有通常促进纤毛膜中浓度的信号的蛋白质（例如 Smoothened）的纤毛定位。没有保留信号的蛋白质（例如绿色荧光蛋白-糖基磷脂酰肌醇）被发现在纤毛中，这表明进入不受扩散障碍或脂质微区的阻碍。因此，相互作用的层次控制着纤毛膜的组成，包括选择性保留、选择性包含和被动扩散。
