Programmed destruction of regulatory proteins through the ubiquitin-proteasome system is a widely used mechanism for controlling signalling pathways. Cullins are proteins that function as scaffolds for modular ubiquitin ligases typified by the SCF (Skp1-Cul1-F-box) complex. The substrate selectivity of these E3 ligases is dictated by a specificity module that binds cullins. In the SCF complex, this module is composed of Skp1, which binds directly to Cul1, and a member of the F-box family of proteins. F-box proteins bind Skp1 through the F-box motif, and substrates by means of carboxy-terminal protein interaction domains. Similarly, Cul2 and Cul5 interact with BC-box-containing specificity factors through the Skp1-like protein elongin C. Cul3 is required for embryonic development in mammals and Caenorhabditis elegans but its specificity module is unknown. Here we report the identification of a large family of BTB-domain proteins as substrate-specific adaptors for C. elegans CUL-3. Biochemical studies using the BTB protein MEL-26 and its genetic target MEI-1 (refs 12, 13) indicate that BTB proteins merge the functional properties of Skp1 and F-box proteins into a single polypeptide.

During C. elegans development, Wnt/WG signaling is required for differences in cell fate between sister cells born from anterior/posterior divisions. A beta-catenin-related gene, wrm-1, and the lit-1 gene are effectors of this signaling pathway and appear to downregulate the activity of POP-1, a TCF/LEF-related protein, in posterior daughter cells. We show here that lit-1 encodes a serine/threonine protein kinase homolog related to the Drosophila tissue polarity protein Nemo. We demonstrate that the WRM-1 protein binds to LIT-1 in vivo and that WRM-1 can activate the LIT-1 protein kinase when coexpressed in vertebrate tissue culture cells. This activation leads to phosphorylation of POP-1 and to apparent changes in its subcellular localization. Our findings provide evidence for novel regulatory avenues for an evolutionarily conserved Wnt/WG signaling pathway.

Cell division in haploid yeast gives rise to a "mother" cell capable of mating-type switching and a "daughter" cell that is not. Switching is initiated by the HO endonuclease, whose gene is only transcribed in cells that have previously given birth to a bud (mother cells). HO expression depends on a minimyosin, She1p/Myo4p, which accumulates preferentially in growing buds. We describe a gene, ASH1, that is necessary to repress HO in daughters. ASH1 encodes a zinc finger protein whose preferential accumulation in daughter cell nuclei at the end of anaphase depends on She1p/Myo4p. The greater abundance of Ash1p in daughter cells is responsible for restricting HO expression to mother cells.

A rapid cascade of regulatory events defines the developmental fates of embryonic cells. However, once established, these developmental fates and the underlying transcriptional programs can be remarkably stable. Here, we describe two proteins, MEP-1 and LET-418/Mi-2, required for maintenance of somatic differentiation in C. elegans. In animals lacking MEP-1 and LET-418, germline-specific genes become derepressed in somatic cells, and Polycomb group (PcG) and SET domain-related proteins promote this ectopic expression. MEP-1 and LET-418 interact in vivo with the germline-protein PIE-1. Our findings support a model in which PIE-1 inhibits MEP-1 and associated factors to maintain the pluripotency of germ cells, while at later times MEP-1 and LET-418 remodel chromatin to establish new stage- or cell-type-specific differentiation potential.