The maternal gene spn-4 encodes a predicted RRM protein required for mitotic spindle orientation and cell fate patterning in early C-elegans embryos

C. elegans embryogenesis begins with a stereotyped sequence of asymmetric c ell divisions that are largely responsible for establishing the nematode bo dy plan. These early asymmetries are specified after fertilization by the w idely conserved, cortically enriched PAR and PKC-3 proteins, which include three kinases and two PDZ domain proteins. During asymmetric cell divisions in the early embryo, centrosome pairs initially are positioned on transver se axes but then rotate to align with the anteroposterior embryonic axis. W e show that rotation of the centrosomal/nuclear complex in an embryonic cel l called P-1 requires a maternally expressed gene we name spn-4. The predic ted SPN-4 protein contains a single RNA recognition motif (RRM), and belong s to a small subfamily of RRM proteins that includes one Drosophila and two human family members. Remarkably, in mutant embryos lacking spn-4 function the transversely oriented 'P-1' mitotic spindle appears to re-specify the axis of cell polarity, and the division remains asymmetric. spn-4 also is r equired for other developmental processes, including the specification of m esendoderm, the restriction of mesectoderm fate to P-1 descendants, and ger mline quiescence during embryogenesis. We suggest that SPN-4 post-transcrip tionally regulates the expression of multiple developmental regulators. Suc h SPN-4 targets might then act more specifically to generate a subset of th e anterior-posterior asymmetries initially specified after fertilization by the more generally required PAR and PKC-3 proteins.