Question

A large ribosomal subunit is on the order of 3200 kDa. The diffusion limit of a nuclear pore is about 40kDa. How do ribosomal subunits ever reach the cytosol? 

A.) Ribosomal subunits are endocytosed into the ER. 

B.) Karyopherin proteins help get them through the pore. 

C.) They eventually diffuse through the nuclear envelope bilayer. 

D.) Ribosomes can't get out, because karyopherins to not bind rRNA.

Answer 1

The nuclear envelope has the inner and outer nuclear membrane. The inner nuclear membrane is the binding site for chromatin and nuclear lamina while the outer nuclear membrane is continuous with the endoplasmic reticulum (ER) membrane.

The ribosome is composed of a large subunit and small subunit. Each of these subunits has ribosomal protein and ribosomal RNA. The ribosomal proteins are synthesized in cytosol and imported to the nucleus. In the nucleus, these ribosomal proteins assemble of the ribosomal proteins to form the subunits. In 60S ribosomal subunit in eukaryotes, the 28S, 5.8S, and 5S rRNA assemble with 50 ribosomal while in the 40S subunit, the 18S rRNA assembles with 33 different proteins.

In the nucleus, RanGTP is present in higher concentration than RanGDP. The export of the large ribosomal is dependent on RanGTP bound karyopherin Crm1p. Nmd3p is the non ribosomal adapter protein between 60S ribosome subunits and the nuclear export by karyopherin Crm1p. Nmd3p binds both 60S subunit and Crm1p. Once it is exported to cytosol, Crm1 is dissociated from Nmd3p by hydrolysis of RanGTP. The cytoplasmic GTPase Lsg1 is involved in this hydrolysis. The large subunit now is released in cytosol.There is no endocytosis involved. Diffusion cannot take place as the larger subunit (32000 KDa) cannot pass on its own through the nuclear pore

Hence, right answer is B) Karyopherin proteins help get them through the pore.