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Publication Abstract from PubMed

The matrix polysaccharide hyaluronan (HA) has a critical role in the expansion of the cumulus cell-oocyte complex (COC), a process that is necessary for ovulation and fertilization in most mammals. Hyaluronan is organized into a cross-linked network by the cooperative action of three proteins, inter-alpha-inhibitor (IalphaI), pentraxin-3, and TNF-stimulated gene-6 (TSG-6), driving the expansion of the COC and providing the cumulus matrix with its required viscoelastic properties. Although it is known that matrix stabilization involves the TSG-6-mediated transfer of IalphaI heavy chains (HCs) onto hyaluronan (to form covalent HC.HA complexes that are cross-linked by pentraxin-3) and that this occurs via the formation of covalent HC.TSG-6 intermediates, the underlying molecular mechanisms are not well understood. Here, we have determined the tertiary structure of the CUB module from human TSG-6, identifying a calcium ion-binding site and chelating glutamic acid residue that mediate the formation of HC.TSG-6. This occurs via an initial metal ion-dependent, non-covalent, interaction between TSG-6 and HCs that also requires the presence of an HC-associated magnesium ion. In addition, we have found that the well characterized hyaluronan-binding site in the TSG-6 Link module is not used for recognition during transfer of HCs onto HA. Analysis of TSG-6 mutants (with impaired transferase and/or hyaluronan-binding functions) revealed that although the TSG-6-mediated formation of HC.HA complexes is essential for the expansion of mouse COCs in vitro, the hyaluronan-binding function of TSG-6 does not play a major role in the stabilization of the murine cumulus matrix.

Metal Ion-dependent Heavy Chain Transfer Activity of TSG-6 Mediates Assembly of the Cumulus-Oocyte Matrix.,Briggs DC, Birchenough HL, Ali T, Rugg MS, Waltho JP, Ievoli E, Jowitt TA, Enghild JJ, Richter RP, Salustri A, Milner CM, Day AJ J Biol Chem. 2015 Nov 27;290(48):28708-23. doi: 10.1074/jbc.M115.669838. Epub, 2015 Oct 14. PMID:26468290^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.