A comparison of the structures of (a) vertebrate and (b) yeast nuclear pore complexes (NPCs). The spoke complex of the vertebrate NPC is composed of three sections: cytoplasmic and nuclear rings (in blue) and the central spoke ring (green; some details of the spoke ring are omitted from the figure, for simplicity, but are described in the text). The yeast NPC lacks the cytoplasmic and nuclear rings. The red structure in the middle of each NPC is the central transporter. The relative sizes of each component are approximate.