[14]. This motif resides in the first231 amino acids of Lig1, whichwere removed for crystalstructure analysis, but Figure 2Billustrates how these two proteinswould be juxtaposed while onDNA. From studies with E. coli[15], the PCNA clamp ispresumed to be left behind on anOkazaki fragment after the DNApolymerase has finishedextending it to a nick. Theabandoned PCNA clamp may actas a marker to recruit Lig1 andmay help orient it as well. Lig1may also employ the slidingclamp to track along DNA until itlocates a nick.Provided Lig1 binds PCNAwhile both proteins encircle DNA,as modeled in Figure 2B, Pascalet al. [3] make the point that Lig1would effectively mask all threeprotein binding sites on the PCNAtrimer, thereby excluding otherproteins from binding the clamp.This steric exclusion arguesagainst the ‘toolbelt’ model inwhich trimeric PCNAsimultaneously binds threedifferent replication proteins [16].For example, instead of FEN1 andLig1 both binding PCNAsimultaneously during Okazakifragment maturation, FEN1 wouldneed to be dislodged from PCNAin order for Lig1 to interact withthe clamp. How FEN1 and Lig1switch places on PCNA, andwhether Lig1 has a specificmechanism to displace FEN1 fromPCNA must await future studies.The structural snapshot of theligase ring about to seal the nickbrings to mind new questions.How does a protein encirclingDNA dissociate after sealing thenick? Perhaps the rigidity of thefully double-stranded DNAproduct provides energy to openthe Lig1 ring. How does Lig1assemble onto DNA? Does theenzyme adenylation stepdestabilize and open the ring?Alternatively, ligase may not be astable ring without DNA. Indeed,the T. filiformis ligase–AMPstructure requires largeconformational changes to bindDNA as a ring. Finally, how doesligase integrate its actions withother binding partners? Thesequestions and many more suggestthat exciting studies of thisfascinating enzyme will continuewell into the future.References1. Kornberg, A., and Baker, T. (1992). DNAreplication, 2nd Edition (New York: W.H.Freeman).2. Lehman, I.R. (1974). DNA ligase:structure, mechanism, and function.Science 186, 790–797.3. Pascal, J.M., O’Brien, P.J., Tomkinson,A.E., and Ellenberger, T. (2004). HumanDNA ligase I completely encircles andpartially unwinds nicked DNA. Nature432, 473–478.4. Subramanya, H.S., Doherty, A.J.,Ashford, S.R., and Wigley, D.B. (1996).Crystal structure of an ATP-dependentDNA ligase from bacteriophage T7. Cell85, 607–615.5. Singleton, M.R., Hakansson, K., Timson,D.J., and Wigley, D.B. (1999). Structureof the adenylation domain of an NAD+-dependent DNA ligase. Structure FoldDes. 7, 35–42.6. Odell, M., Sriskanda, V., Shuman, S., andNikolov, D.B. (2000). Crystal structure ofeukaryotic DNA ligase-adenylateilluminates the mechanism of nicksensing and strand joining. Mol. Cell 6,1183–1193.7. Lee, J.Y., Chang, C., Song, H.K., Moon,J., Yang, J.K., Kim, H.K., Kwon, S.T., andSuh, S.W. (2000). Crystal structure ofNAD(+)-dependent DNA ligase: modulararchitecture and functional implications.EMBO J. 19, 1119–1129.8. Tomkinson, A.E., Tappe, N.J., andFriedberg, E.C. (1992). DNA ligase I fromSaccharomyces cerevisiae: physical andbiochemical characterization of theCDC9 gene product. Biochemistry 31,11762–11771.9. Shuman, S. (1995). Vaccinia virus DNAligase: specificity, fidelity, and inhibition.Biochemistry 34, 16138–16147.10. Levin, D.S., Bai, W., Yao, N., O’Donnell,M., and Tomkinson, A.E. (1997). Aninteraction between DNA ligase I andproliferating cell nuclear antigen:implications for Okazaki fragmentsynthesis and joining. Proc. Natl. Acad.Sci. USA 94, 12863–12868.11. Montecucco, A., Rossi, R., Levin, D.S.,Gary, R., Park, M.S., Motycka, T.A.,Ciarrocchi, G., Villa, A., Biamonti, G., andTomkinson, A.E. (1998). DNA ligase I isrecruited to sites of DNA replication byan interaction with proliferating cellnuclear antigen: identification of acommon targeting mechanism for theassembly of replication factories. EMBOJ. 17, 3786–3795.12. Lopez de Saro, F.J., and O’Donnell, M.(2001). Interaction of the beta slidingclamp with MutS, ligase, and DNApolymerase I. Proc. Natl. Acad. Sci. USA98, 8376–8380.13. Warbrick, E. (2000). The puzzle ofPCNA’s many partners. Bioessays 22,997–1006.14. Bambara, R.A., Murante, R.S., andHenricksen, L.A. (1997). Enzymes andreactions at the eukaryotic DNAreplication fork. J. Biol. Chem. 272,4647–4650.15. Stukenberg, P.T., Turner, J., andO’Donnell, M. (1994). An explanation forlagging strand replication: polymerasehopping among DNA sliding clamps. Cell78, 877–887.16. Maga, G., and Hubscher, U. (2003).Proliferating cell nuclear antigen (PCNA):a dancer with many partners. J. Cell Sci.116, 3051–3060.17. Gulbis, J.M., Kelman, Z., Hurwitz, J.,O’Donnell, M., and Kuriyan, J. (1996).Structure of the C-terminal region ofp21(WAF1/CIP1) complexed with humanPCNA. Cell 87, 297–306.1Howard Hughes Medical Institute, 2TheRockefeller University 1230 YorkAvenue, Box 228, New York, New York10021-6399, USA.E-mail: [email protected]: 10.1016/j.cub.2005.01.025Current Biology Vol 15 No 3R92Seth S. BlairThe proteoglycans are a majorcomponent of cell surfaces andthe extracellular matrix [1]. Theyare made from a core proteindecorated with one or moreglycosaminoglycan side chains,unbranched carbohydratepolymers made of disaccharidesubunits. The two major familiesof cell surface proteoglycans arethe transmembrane Syndecans,which are decorated with theglycosaminoglycans heparansulfate and chondroitin sulfate,and the Glypicans, which aredecorated with heparan sulfateand are anchored to the cellsurface via a glycosylphos-phatidylinositol (GPI) linkage.Proteoglycans play a number ofdifferent roles, but one of the mostintriguing is the regulation ofsignaling between cells. Specificglycosaminoglycans can recognizeand bind members of severaldifferent families of signals and, inhumans, defects in Glypicansinduce Simpson-Golabi-BehmelCell Signaling: Wingless andGlypicans Together AgainThe role of the Glypican proteoglycans in Wingless signaling has beencontroversial. New studies show that the Glypican Dally-like can haveboth positive and negative effects on Wingless signaling; moreover,signaling can be regulated by removing Dally-like from the cell surface.syndrome, associated with tissueovergrowth. A number of researchers havetherefore been examining howproteoglycans regulatedevelopmental signals. Examplesinclude the Syndecans, which canform a complex with