Answer:
The pores remain closed until they are stimulated by the binding of protein with the proper localization signal.
Explanation:
The pores are constructed from a class of proteins called “nucleoporins,” a subset of which contains a tandem series of phenylalanine-glycine (FG) repeats that line the central transport channel of the pore . The inbound proteins are captured by the nuclear basket and released by GTP hydrolysis. The cytosolic fibrils obstruct access to the pore and can be parted by nuclear import receptors. Thus, the pores remain closed until they are stimulated by the binding of protein with the proper localization signal.
Allen, T. D., Cronshaw, J. M., Bagley, S., Kiseleva, E., and Goldberg, M. W.(2000) J. Cell Sci. 113, 1651–165
