Program Director/Principal Investigator (Last, First, Middle): Song, Mi Hye
Summary
Centrosomes, as the primary microtubule-organizing center, establish bipolar spindles that ensure accurate
transmission of genetic contents into two daughter cells. To maintain genomic integrity, centrosome number
must be strictly regulated by duplicating only once per cell cycle. Abnormal centrosomes are associated with
human disorders, including cancers, microcephaly and other developmental defects. Our long-term goal is to
understand the genetic mechanisms of centrosome function and assembly using the early C. elegans embryo
as an in vivo model. The overall objective is to investigate how the ubiquitin ligase, anaphase promoting
complex/cyclosome (APC/C) and the coactivator FZR-1 (Cdh1 in human), contributes to centrosome assembly
through proteasomal degradation of centrosome factors. FZR-1/Cdh1 (an APC/C cofactor) confers a substrate-
binding through recognizing highly conserved degron motifs (KEN- and D-boxes). Cdh1-deficient mice exhibit
embryonic lethality, genomic instability, and higher susceptibility to tumorigenesis and defective brain
development. The abundance of centrosome components directly influences centrosome number: Blocking
degradation of centrosome factors causes extra centrosomes, while depletion inhibits centrosome duplication.
While we realize the great impact of the APC/C complex for regulating levels of centrosome factors, its substrate
repertoire and regulatory mechanisms remain elusive. We propose to investigate SAS-7 as a potential substrate
of APC/CFZR-1: SAS-7 functions most upstream in centrosome assembly and SAS-7 protein contains conserved
degron motifs at multiple sites. We hypothesize that SAS-7 is another centrosome factor that is directly
targeted APC/CFZR-1. Inhibiting APC/CFZR-1 blocks SAS-7 degradation, leading to hyper-stabilization of SAS-7
and compensating for a partial loss of ZYG-1 function in zyg-1 mutants. Our rationale is that its substrate
repertoire of APC/CFZR-1 in centrosome assembly and defining their role will reveal the regulatory mechanisms
of APC/CFZR-1 required for the fidelity of cell division. The aims of the project are to (1) determine if SAS-7 levels
are affected by APC/CFZR-1-dependent proteolysis, (2) identify functional degron motifs within SAS-7, mediating
APC/CFZR-1 targeting, and (3) understand how APC/CFZR-1-dependent proteolytic regulation of SAS-7
contributes to centrosome assembly and function. The use of in vivo-based genetics in a model system C.
elegans will lay the groundwork for understanding human systems. The proposed project should contribute to
advances in fundamental understandings of centrosome biology in humans and therapeutic interventions for
human diseases and conditions such as cancers and microcephaly associated with abnormal centrosomes.