NPC

The structure of the nuclear pore complex and its role in nucleoplasmic exchange
The nuclear envelope is the barrier between the nucleus and cytoplasm, and nuclear pores are the gateways across that barrier.
Unlike the PM, which prevents passage of macromolecules between the cytoplasm and the extracellular space, the nuclear envelope is a hub of activity for the movement of RNAs and protein in both directions between the nucleus and cytoplasm.
The replication and transcription of genetic material within the nucleus require the participation of large numbers of proteins that are synthesized in the cytoplasm and transported across the nuclear envelope.
Conversely, the m-RNAs, t-RNAs, and ribosomal subunits that are manufactured in the nucleus must be transported through the nuclear envelope in the opposite direction.
Some components, such as the SnRNAs (small nuclear RNA) of the Splicesome (A macromolecular complex containing a variety of proteins and a number of distinct ribonucleoproteins particles that functions in removal of intron from a primary transcript), move in both directions; they are synthesized in the nucleus, assembled into RNP(ribonucleo protein) particles in the cytoplasm and then shipped back to the nucleus where they function in mRNA processing.
To appreciate the magnitude of the traffic between two major cellular compartments, consider a HeLa cell, which is estimated to contain about 10,000.000 ribosomes.
To support its growth, a single HeLa cell nucleus must import approximately 560,000 ribosomal proteins and export approximately 14,000 ribgosomal subunits every minute.

















STRUCTURE OF NPC (NUCLEAR PORE COMPLEX)-

Nuclear pores contain a complex basket like apparatus called the NPC
The NPC is a huge supramolecular complex-15 to 30 times the mass of a ribosome that exhibits octagonal symmetry due to the eight fold repetition of a number of structures.
NPCs contaion only about 30 different proteins, called nucleoporins.
Each nucleoporins is present in at least eight copies, in keeping with the octagonal symmetry of the structure.
The position of the various nucleoporins within the NPC has been observed under the electron microscope using gold labeled antibodies.





Molecular trafficking between the nucleus and the cytoplasm of interphase cells occurs via the nuclear pore complexes (NPCs), large supramolecular assemblies that are embedded in the double-membraned nuclear envelope (NE). The NPCs provide peripheral channels of about 9 nm in diameter, which allow the diffusion of ions and small molecules, and mediate the selective transport of nuclear proteins, RNAs, and ribonucleoprotein (RNP) particles by energy-dependent mechanisms