Project Summary
The eukaryotic nucleus is enclosed by a double-membrane structure, the nuclear envelope, which
separates the nucleoplasm from the cytoplasm. The outer nuclear membrane is continuous with the
endoplasmic reticulum (ER), whereas the inner nuclear membrane (INM) is a specialized cellular compartment
with a unique proteome. In order to ensure compartmental homeostasis, the INM-associated degradation
(INMAD) pathway is required for both protein quality control and regulated proteolysis of INM proteins. We
have recently discovered a novel INMAD branch functioning through the E3 ubiquitin ligase: the anaphase
promoting complex/cyclosome (APC/C), which degrades the integral INM protein Mps3 in budding yeast.
Furthermore, APC/C-dependent INMAD regulates nuclear envelope reorganization, a key factor controlling
replicative lifespan and cell aging. Our hypothesis is that APC/C directly ubiquitinates INM substrates,
targeting them for proteasomal degradation, which process is critical for maintaining nuclear envelope
homeostasis. Three specific aims will be fulfilled to test this hypothesis: (1) determine the modes of APC/C-
dependent INMAD substrate recognition, (2) determine the modes of APC/C-dependent INMAD substrate
extraction and translocation, and (3) determine the pathological consequences of impaired APC/C-mediated
INMAD. Under the first aim, the critical cis and trans factors responsible for APC/C-mediated INM protein
ubiquitination and degradation will be characterized. Under the second aim, the mechanism by which the
AAA-ATPase Cdc48/p97 and its cofactors extract and translocate APC/C-dependent INMAD substrates will be
characterized. Under the third aim, the biological function of nuclear envelope remodeling and replicative
lifespan regulated by APC/C-mediated INMAD will be determined. The proposed research is expected to
provide key insights into the mechanism controlling APC/C-dependent INMAD substrate ubiquitination,
translocation and degradation, and the pathological consequences of INMAD impairment. Aggregation of
SUN1, the Mps3 homolog, has been implicated in premature cell aging and dystrophic laminopathies in
mammals. Unraveling how APC/C-mediated INMAD regulates its substrate turnover in yeast is a crucial step to
understanding the molecular mechanisms governing INM protein turnover in human cells and may provide a
direct link between nuclear envelope-associated diseases and cell-cycle regulation.