We tested the hypothesis that gp210, an integral membrane protein of nuclear 
pore complexes (NPCs), mediates nuclear pore formation. Gp210 has a large 
lumenal domain and small COOH-terminal tail exposed to the cytoplasm. We studied 
the exposed tail. We added recombinant tail polypeptides to Xenopus nuclear 
assembly extracts, or inhibited endogenous gp210 tails using anti-tail 
antibodies. Both strategies had no effect on the formation of fused flattened 
nuclear membranes, but blocked NPC assembly and nuclear growth. Inhibited nuclei 
accumulated gp210 and some nucleoporin p62, but failed to incorporate 
nup214/CAN, nup153, or nup98 and were defective for nuclear import of lamin B3. 
Scanning and transmission EM revealed a lack of "closely apposed" inner and 
outer membranes, and the accumulation of novel arrested structures including 
"mini-pores." We conclude that gp210 has early roles in nuclear pore formation, 
and that pore dilation is mediated by gp210 and its tail-binding partner(s). We 
propose that membrane fusion and pore dilation are coupled, acting as a 
mechanism to control nuclear pore size.