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J. Cell Biol.,
Volume 144, Number 5, March 8, 1999 839-855
* Laboratory of Cell Biology, Howard Hughes Medical Institute; and While much has been learned in recent years
about the movement of soluble transport factors across
the nuclear pore complex (NPC), comparatively little is
known about intranuclear trafficking. We isolated the
previously identified Saccharomyces protein Mlp1p (myosin-like protein) by an assay designed to find nuclear envelope (NE) associated proteins that are not
nucleoporins. We localized both Mlp1p and a closely
related protein that we termed Mlp2p to filamentous
structures stretching from the nucleoplasmic face of the
NE into the nucleoplasm, similar to the homologous
vertebrate and Drosophila Tpr proteins. Mlp1p can be
imported into the nucleus by virtue of a nuclear localization sequence (NLS) within its COOH-terminal domain. Overexpression experiments indicate that Mlp1p
can form large structures within the nucleus which exclude chromatin but appear highly permeable to proteins. Remarkably, cells harboring a double deletion of
MLP1 and MLP2 were viable, although they showed a
slower net rate of active nuclear import and faster passive efflux of a reporter protein. Our data indicate that
the Tpr homologues are not merely NPC-associated
proteins but that they can be part of NPC-independent,
peripheral intranuclear structures. In addition, we suggest that the Tpr filaments could provide chromatin-free conduits or tracks to guide the efficient translocation of macromolecules between the nucleoplasm and
the NPC.
Laboratory of Cellular and Structural Biology, The
Rockefeller University, New York, New York 10021
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