What are NPCs primarily composed of?

Nuclear pore complexes (NPCs) are primarily composed of a group of proteins known as nucleoporins. These proteins form the structural framework of the NPC, playing critical roles in the transport of molecules between the nucleus and the cytoplasm. Nucleoporins have various functions, including forming the selective barrier of the nuclear pore, facilitating nuclear transport, and contributing to the overall stability of the NPC structure.

The other options do not accurately describe the primary components of NPCs. Nuclear pores are the structures themselves that are composed of nucleoporins, while ribosomal subunits and cytoskeletal proteins play different roles within the cell. Ribosomal subunits are involved in protein synthesis, and cytoskeletal proteins are critical for maintaining cell shape and enabling movement. Thus, understanding the role of nucleoporins helps clarify the composition and function of NPCs in cellular processes.