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The Journal of Cell Biology, Vol 117, 1277-1287, Copyright © 1992 by The Rockefeller University Press

ARTICLES

Separate domains of KAR1 mediate distinct functions in mitosis and nuclear fusion

EA Vallen, MA Hiller, TY Scherson and MD Rose
Department of Molecular Biology, Lewis Thomas Laboratory, Princeton University, New Jersey 08544-1014.

The yeast KAR1 gene is essential for mitotic growth and important for nuclear fusion. Mutations in KAR1 prevent duplication of the spindle pole body (SPB), and affect functions associated with both the nuclear and cytoplasmic microtubules. The localization of hybrid Kar1-lacZ proteins, described elsewhere (Vallen, E. A., T. Y. Scherson, T. Roberts, K. van Zee, and M. D. Rose. 1992. Cell. In press), suggest that the protein is associated with the SPB. In this paper, we report a deletion analysis demonstrating that the mitotic and karyogamy functions of KAR1 are separate and independent, residing in discrete functional domains. One region, here shown to be essential for mitosis, coincided with a part of the protein that is both necessary and sufficient to target Karl-lacZ hybrid proteins to the SPB (Vallen, E. A., T. Y. Scherson, T. Roberts, K. van Zee, and M. D. Rose. 1992. Cell. In press). Complementation testing demonstrated that deletions in this interval did not affect nuclear fusion. A second region, required only for karyogamy, was necessary for the localization of a Kar3-lacZ hybrid protein to the SPB. These data suggest a model for the roles of Kar1p and Kar3p, a kinesin-like protein, in nuclear fusion. Finally, a third region of KAR1 was found to be important for both mitosis and karyogamy. This domain included the hydrophobic carboxy terminus and is sufficient to target a lacZ-Kar1 hybrid protein to the nuclear envelope (Vallen E. A., T. Y. Scherson, T. Roberts, K. van Zee, and M. D. Rose. 1992. Cell. In press). Altogether, the essential mitotic regions of KAR1 comprised 20% of the coding sequence. We propose a model for Kar1p in which the protein is composed of several protein-binding domains tethered to the nuclear envelope via its hydrophobic tail.
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