Identification of novel suppres sors of crumbs and their role in nuclear division and growth regulation

Abstract

Crumbs (Crb) family proteins are crucial for cell polarity. Recent studies indicate that they are also involved in growth regulation and cancer. However, it is not well understood how Crb participates in mitotic processes. Here I report that Crb is critically involved in nu-clear division by interacting with Xeroderma pigmentosum D (XPD). A novel gene named gallajin-1 (galla-1) was identified from a genetic screen for crb modifiers. Galla-1 protein shows homology to MIP18, a subunit of the mitotic spindle-associated MMS19-XPD com-plex. Loss-of-function galla-1 mutants show abnormal chromosome segregation, defective centrosome positions, and branched spindles during nuclear division in early embryos. Em-bryos with loss-of-function or overexpression of crb show similar mitotic defects and genetic interaction with galla-1. Both Galla-1 and Crb protein show overlapping localization with spindle microtubules during nuclear division. Galla-1 physically interacts with the intracellular domain of Crb, but shows little binding to the Drosophila homolog of XPD. I identified a related protein Galla-2 that binds both Crb and Xpd. Loss-of-function galla-2 mutants show similar mitotic defects as galla-1 and strong genetic interaction with crb. Xpd can form a complex with Crb via Galla-2 but also by direct binding. Crb, Galla and Xpd share mutant phenotypes, and show genetic as well as physical interactions. This study reveals that Crb is required for proper chromosome segregation during nuclear division by forming a novel com-plex with Galla and Xpd, implicating a potential link between Crb and Xpd-related genome instability.