STING functions as both an adaptor protein signaling cytoplasmic double-stranded DNA and a direct immunosensor of cyclic diguanylate monophosphate (c-di-GMP). The crystal structures of the C-terminal domain of human STING (STING(CTD)) and its complex with c-di-GMP reveal how STING recognizes c-di-GMP. In response to c-di-GMP binding, two surface loops, which serve as a gate and latch of the cleft formed by the dimeric STING(CTD), undergo rearrangements to interact with the ligand.

CUL4A and CUL4B, which are derived from the same ancestor, CUL4, encode scaffold proteins that organize cullin-RING ubiquitin ligase (E3) complexes. Recent genetic studies have shown that germ line mutation in CUL4B can cause mental retardation, short stature, and other abnormalities in humans. CUL4A was observed to be overexpressed in breast and hepatocellular cancers, although no germ line mutation in human CUL4A has been reported. Although CUL4A has been known to be involved in a number of cellular processes, including DNA repair and cell cycle regulation, little is known about whether CUL4B has similar functions. In this report, we tested the functional importance of CUL4B in cell proliferation and characterized the nuclear localization signal (NLS) that is essential for its function. We found that RNA interference silencing of CUL4B led to an inhibition of cell proliferation and a prolonged S phase, due to the overaccumulation of cyclin E, a substrate targeted by CUL4B for ubiquitination. We showed that, unlike CUL4A and other cullins that carry their NLS in their C termini, NLS in CUL4B is located in its N terminus, between amino acid 37 and 40, KKRK. This NLS could bind to importin ␣1, ␣3, and ␣5. NLS-deleted CUL4B was distributed in cytoplasm and failed to promote cell proliferation. Therefore, the nuclear localization of CUL4B mediated by NLS is critical for its normal function in cell proliferation.Cullins function as a "scaffold" in cullin-RING-based E3 ubiquitin ligases (CRLs).3 CRLs constitute a major subclass of RING finger E3s that regulate diverse cellular processes, including cell cycle progression, transcription, signal transduction, and development (1, 2). Cullins are evolutionarily conserved from yeast to mammals. Although sequence homology spans the entire protein, the C terminus, characterized by the ϳ200-amino acid cullin homology domain, is most conserved. Humans encode seven cullin members, CUL1, CUL2, CUL3, CUL4A, CUL4B, CUL5, and CUL7 (2). CUL4A and CUL4B are derived from one ancestor, CUL4, which exists in Schizosaccharomyces pombe (Pcu4), Xenopus laevis, Caenorhabditis elegans, Drosophila melanogaster, and Arabidopsis thaliana but is absent in Saccharomyces cerevisiae. CUL4A and CUL4B also exist in other higher organisms, including zebrafish and the mouse (3). The protein sequences between human CUL4A and CUL4B are 83% identical, with CUL4B having a unique N terminus of 149 amino acids (supplemental Fig. S1). CUL4A CRL complexes were shown to contain Rbx1 and the adaptor protein DDB1. DDB1 interacts with WD-40 repeat motif-containing proteins that determine the substrate specificity of the CUL4A ubiquitin ligase complex (3-7). Loss of Cul4 in Drosophila cells leads to G 1 arrest that is associated with an increase in the cyclin-dependent kinases (CDK) inhibitor Dacapo (8). Recent genetic studies have shown that mutations in CUL4B gene can cause an X-linked mental retardation syndrome (9, 10). However, no germ line mutation in human CUL4A has been reported, although overexpression of ...

The GIPC family adaptor proteins mediate endocytosis by tethering cargo proteins to the myosin VI motor. The structural mechanisms for the GIPC/cargo and GIPC/myosin VI interactions remained unclear. PlexinD1, a transmembrane receptor that regulates neuronal and cardiovascular development, is a cargo of GIPCs. GIPC-mediated endocytic trafficking regulates PlexinD1 signaling. Here, we unravel the mechanisms of the interactions among PlexinD1, GIPCs and myosin VI by a series of crystal structures of these proteins in apo or bound states. GIPC1 forms a domain-swapped dimer in an autoinhibited conformation that hinders binding of both PlexinD1 and myosin VI. PlexinD1 binding to GIPC1 releases the autoinhibition, promoting its interaction with myosin VI. GIPCs and myosin VI interact through two distinct interfaces and form an open-ended alternating array. Our data support that this alternating array underlies the oligomerization of the GIPC/Myosin VI complexes in solution and cells.DOI: http://dx.doi.org/10.7554/eLife.27322.001

No abstract

Cyclic dinucleotides are a newly expanded class of second messengers that contribute to the regulation of multiple different pathways in bacterial, eukaryotic, and archaeal cells. The recently identified Vibrio cholerae dinucleotide cyclase (DncV, the gene product of VC0179) can generate three different cyclic dinucleotides and preferentially synthesize a hybrid cyclic-GMP-AMP. Here, we report the crystal structural and functional studies of DncV. We unexpectedly observed a 5-methyltetrahydrofolate diglutamate (5MTHFGLU2) molecule bound in a surface pocket opposite the nucleotide substrate-binding groove of DncV. Subsequent mutagenesis and functional studies showed that the enzymatic activity of DncV is regulated by folate-like molecules, suggesting the existence of a signaling pathway that links folate-like metabolism cofactors to the regulation of cyclic dinucleotide second messenger synthesis. Sequence analysis showed that the residues involved in 5MTHFGLU2 binding are highly conserved in DncV orthologs, implying the presence of this regulation mechanism in a wide variety of bacteria.