Qualitative highly divergent nuclear export signals can regulate export bythe competition for transport cofactors in vivo

Nucleo-cytoplasmic transport of proteins is mediated by nuclear export sign als, identified in various proteins executing heterologous biological funct ions. However, the molecular mechanism underlying the orchestration of expo rt is only poorly understood. Using microinjection of defined recombinant e xport substrates, we now demonstrate that leucine-rich nuclear export signa ls varied dramatically in determining the kinetics of export in vivo. Thus, nuclear export signals could be kinetically classified which correlated wi th their affinities for CRM1-containing export complexes in vitro. Striking ly, cotransfection experiments revealed that proteins containing a fast nuc lear export signal inhibited export and the biological activity of proteins harboring a slower nuclear export signal in vivo. The affinity for export complexes seems therefore predominantly controlled by the nuclear export si gnal itself, even in the context of the complete protein in vivo. Overexpre ssion of FG-rich repeats of nucleoporins affected a medium nuclear export s ignal containing protein to the same extent as a fast nuclear export signal containing protein, indicating that nucleoporins appear not to contribute significantly to nuclear export signal-specific export regulation. Our resu lts imply a novel mode for controlling the biological activity of shuttle p roteins already by the composition of the nuclear export signal itself.