What allows the passage of proteins from the cytoplasm into the nucleus?

The nuclear pore complex serves as a gateway for the transport of proteins and other macromolecules between the cytoplasm and the nucleus. It is a large protein complex embedded in the nuclear envelope, which encircles the nucleus. The nuclear pore complex is composed of multiple proteins that collectively form channels through which specific proteins can pass.

The transport through these pores is highly regulated and selective. Proteins that are meant to enter the nucleus typically contain a nuclear localization signal (NLS), which is recognized by transport proteins called importins. These importins facilitate the translocation of the cargo proteins through the nuclear pore complex into the nucleus.

In contrast, planar diffusion does not adequately explain how larger molecules like proteins can cross cellular membranes, given their size and the structural characteristics of lipid bilayers. Membrane channels, while they do allow transport, typically function for ions and small molecules rather than larger protein complexes. Endocytosis is a process for engulfing materials from the outside of the cell into vesicles, and it does not pertain to the transport of proteins directly into the nucleus. Thus, the nuclear pore complex is the essential structure that allows for the regulated passage of proteins into the nucleus, making it the correct answer.