STRIPAK regulation of katanin microtubule severing in the Caenorhabditis elegans embryo

Abstract

Microtubule severing plays important roles in cell structure and cell division. The microtubule severing protein katanin, composed of the MEI-1/MEI-2 subunits in C. elegans, is required for oocyte meiotic spindle formation; however, it must be inactivated for mitosis to proceed as continued katanin expression is lethal. Katanin activity is regulated by two ubiquitin-based protein degradation pathways. Another ubiquitin ligase, HECD-1, the homolog of human HECTD1/HECT domain E3 ubiquitin protein ligase 1, regulates katanin activity without affecting katanin levels. In other organisms, HECD-1 is a component of the striatin-interacting kinase phosphatase (STRIPAK) complex, which affects cell proliferation and a variety of signaling pathways. Here we conducted a systematic screen of how mutations in STRIPAK components affect katanin function in C. elegans. STRIPAK core components (FARL-11, CASH-1, LET-92, GCK-1) were katanin inhibitors in mitosis and activators in meiosis, much like HECD-1. By contrast, variable components (SLMP-1, OTUB-2) functioned as activators of katanin activity in mitosis, indicating they may function to alter STRIPAK core function. The core component CCM-3 acted as an inhibitor at both divisions, while other components (MOB-4, C49H3.6) showed weak interactions with katanin mutants. Additional experiments indicate that katanin may be involved with the centralspindlin complex and a tubulin chaperone. HECD-1 shows ubiquitous expression in the cytoplasm throughout meiosis and early development. The differing functions of the different subunits could contribute to the diverse functions of the STRIPAK complex in C. elegans and other organisms.