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First published online September 26, 2003
doi: 10.1242/10.1242/jcs.00817

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p97, a protein coping with multiple identities

School of Biological Sciences, University of Manchester, Oxford Road, Manchester M13 9PT, UK

View larger version (29K): [in a new window]   Fig. 1. Functions of p97/Cdc48p. Some functions of p97 are described, including those that occur in interphase (top) and those that are restricted to telophase (bottom). The appropriate adaptor for each activity (where known) is identified. (A) Retrotranslocation of unfolded proteins from the endoplasmic reticulum (ER); unfolded proteins are passed back through the Sec61 translocon (green) and released by p97 so that they can be degraded by the proteasome. (B) Formation of tER; p97 supports homotypic fusion of smooth ER membranes required to form post-ER transport intermediates. (C) Activation of SPT23; the proteasome degrades the C-terminal portion of SPT23, whilst p97 releases the N-terminal product from its full-length SPT23-binding partner so that it can be transported to the nucleus. (D) Nuclear factor B (NFB) activation; p97 cooperates in an unknown way with the proteasome to degrade the regulatory factor IB, allowing NFB (pink) to enter the nucleus. (E) Reformation of Golgi cisternae from tubulo-vesicular clusters occurs as the cell exits mitosis [note that another AAA protein, N-ethylmaleimide-sensitive fusion protein (NSF), is required in addition to p97 for full Golgi stack reassembly]. (F,G) Nuclear envelope reassembly; this occurs in two phases, resealing of the nuclear envelope (requiring Ufd1–Npl4–p97) and expansion (requiring p47–p97).

View larger version (31K): [in a new window]   Fig. 2. AAA proteins involved in membrane fusion. (A) NSF (N-ethylmaleimide-sensitive fusion protein) acts to separate cis SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein proteins) complexes formed as a consequence of membrane fusion. The freed v-SNAREs (blue) are recycled so that they can be incorporated into new transport vesicles and thereby eventually form trans SNARE complexes with t-SNAREs (red) in the target membrane. (B) p97 separates t-SNARE complexes on post-mitotic membranes, allowing subsequent trans SNARE pairing and membrane fusion. Ubiquitinated cofactors may regulate SNARE disassembly by helping to recruit p47–p97. (C,D) Two possible modes of ubiquitin-dependent multivesicular body (MVB) formation. In C, ubiquitinated cargo recruits ESCRT-I, which recruits/activates ESCRT-II and ESCRT-III complexes, forming a domain in the endosome membrane that restricts cargo and selects it for incorporation into internal vesicles (ESCRT-III recruitment is linked to deubiquitination). The ESCRT-III complex is recycled by VPS4. In D, recruited ESCRT-III may act directly during internal vesicle invagination in an analogous way to SNARE-mediated membrane fusion. Again, VPS4 is required for ESCRT-III recycling.

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