Nuclear import substrates compete for a limited number of binding sites. Evidence for different classes of yeast nuclear import receptors.

A nuclear receptor likely involved in nuclear protein import is described. Purified ATP-depleted yeast nuclei show saturable high-affinity binding of the yeast nuclear protein Mcm1. The dissociation constant for the binding is 0.5 microM, and the number of binding sites is approximately 3,500 per nucleus, equivalent to 10-30 binding sites per nuclear pore. Mcm1 competes with other yeast nuclear proteins Ste12 and Swi5, but not with Rap1 or Nop1, indicating that there may be different types of import receptors. Bound Mcm1 is resistant to extraction by nucleases, salt, and non-ionic detergent, but can be released by 5 M urea, suggesting that Mcm1 binds to a yeast equivalent of the nuclear pore complex-lamina fraction of higher eukaryotes.