The nucleus has an inner and an outer membrane and is perforated through 3000-4000 nuclear pores. Each pore having of a nuclear pore complex of more than 100 several proteins organized in a hexagonal array.  Whereas little  molecules  can  pass  by  the  pore  by  free  diffusion and  large  proteins entering  the nucleus need a nuclear localization  signal. That is four to eight amino  acids long and is rich in the positively  charged  amino  acids arginine and lysine as well as commonly  having  proline,  and is situated  internally  within the  polypeptide   chain.  The protein is taken by the pore in an ATP- requires step and enters the nucleus without cleavage of the localization signal.

                         Figure:  Uptake of proteins into the mitochondrial matrix