title: Reactivity of free thiol groups in type-I inositol trisphosphate receptors.
abstract: The IP3R (inositol 1,4,5-trisphosphate receptor) Ca2+-release channel is known to be sensitive to thiol redox state. The present study was undertaken to characterize the number and location of reactive thiol groups in the type-I IP3R. Using the fluorescent thiol-reactive compound monobromobimane we found that approx. 70% of the 60 cysteine residues in the type-I IP3R are maintained in the reduced state. The accessibility of these residues was assessed by covalently tagging the IP3R in membranes with a 5 kDa or 20 kDa MPEG [methoxypoly(ethylene glycol) maleimide]. MPEG reaction caused a shift in the mobility of IP3R on SDS/PAGE that was blocked by pretreatment of the membranes with dithiothreitol, N-ethylmaleimide, mersalyl or thimerosal, indicating that MPEG reactivity was specific to thiol groups on the IP3R. Trypsin cleavage of the type-I IP3R generates five defined domains. In cerebellum membranes, MPEG reacted over a 5 min interval with tryptic fragment I and fragment III, but not fragments II, IV or V. Fragment I appears as a doublet in cerebellum membranes, corresponding to the presence and absence of the SI splice site in this region (SI is a spliced domain corresponding to amino acids 318-332). Only the fragment I band corresponding to the SI(+) splice form shifted after reaction with MPEG. Expression of SI(+) and SI(-) spliced forms in COS cell microsomes confirmed this result. The MPEG-induced shift was not prevented when the cysteine residue present in the SI splice domain (C326A) or the remaining seven cysteine residues in fragment I were individually mutated. Of the combination mutations screened, only the mutation of C206/214/326A blocked MPEG reactivity in fragment I. We conclude that a set of highly reactive cysteine residues in fragment I are differentially accessible in the SI(+) and SI(-) splice variants of the type-I IP3R.
authors.affiliation: Department of Pathology, Thomas Jefferson University, Room 230A JAH, 1020 Locust Street, Philadelphia, PA 19107, USA. suresh.joseph@mail.tju.edu
authors.forename: Suresh K
authors.initials: SK
authors.lastname: Joseph
authors.forename: Steven K
authors.initials: SK
authors.lastname: Nakao
authors.forename: Siam
authors.initials: S
authors.lastname: Sukumvanich
mesh_terms: D017398:Alternative Splicing; D000818:Animals; D019556:COS Cells; D015220:Calcium Channels / Q000737:chemistry* / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D002531:Cerebellum; D002522:Chlorocebus aethiops; D053496:Inositol 1,4,5-Trisphosphate Receptors; D007425:Intracellular Membranes / Q000378:metabolism; D008861:Microsomes / Q000378:metabolism; D051381:Rats; D018160:Receptors, Cytoplasmic and Nuclear / Q000737:chemistry* / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D013438:Sulfhydryl Compounds / Q000737:chemistry / Q000378:metabolism*
chemical_list: D015220:Calcium Channels; D053496:Inositol 1,4,5-Trisphosphate Receptors; D018160:Receptors, Cytoplasmic and Nuclear; D013438:Sulfhydryl Compounds