No abstract

We also compared the effects of the these IP 3 Ks with other enzymes that metabolize Ins(1,4,5)P 3 , including the Type I Ins(1,4,5)P 3 5-phosphatase, in both membrane-targeted and soluble forms, the human inositol polyphosphate multikinase, and the two isoforms of IP 3 K found in Drosophila. All reduce the Ca 2؉ signal but to varying degrees. We demonstrate that the activity of only one of two IP 3 K isoforms from Drosophila is positively regulated by calmodulin and that neither isoform associates with the cytoskeleton. Together the data suggest that IP 3 Ks evolved to regulate kinetic and spatial aspects of Ins (1,4,5)P 3 signals in increasingly complex ways in vertebrates, consistent with their probable roles in the regulation of higher brain and immune function.The metabolism of the calcium-mobilizing second messenger inositol 1,4,5-trisphosphate (Ins(1,4,5)P 3 ) 5 proceeds via two biochemical pathways. Type I Ins(1,4,5)P 3 5-phosphatase catalyzes the dephosphorylation of Ins(1,4,5)P 3 , producing the inactive (1, 2) metabolite Ins(1,4)P 2 (3). Alternatively, Ins(1,4,5)P 3 can be further phosphorylated to Ins(1,3,4,5)P 4 , in a reaction catalyzed by Ins(1,4,5)P 3 3-kinase (IP 3 K) (4). Ins(1,3,4,5)P 4 does not mobilize Ca 2ϩ , but is suggested to have a number of second messenger functions, most notably in the immune system (5, 6) and in neurons (7, 8) (reviewed in Ref. 9). Furthermore, IP 3 Ks may initiate a chain of reactions responsible for the production of higher inositol phosphates, having diverse intracellular roles (10), although this view has been challenged (11, 12).The human genome encodes three differentially expressed isoforms of IP 3 K, denoted IP 3 KA, -B, and -C (13, 14). In addition, inositol polyphosphate multikinase (IPMK; also described as IPK2, and Arg82 in yeast) harbors a range of inositol phosphate kinase activities, including Ins (1,4,5)P 3 3-kinase activity (15, 16). Two IP 3 Ks occur in Drosophila melanogaster, denoted dmIP 3 K␣ (17) and dmIP 3 K␤ (11). One IP 3 K gene occurs in Caenorhabditis elegans, and genetic evidence from both worms and flies strongly argues that IP 3 Ks function as negative regulators of Ins(1,4,5)P 3 Ca 2ϩ signals through their ability to remove Ins(1,4,5)P 3 (17, 18). Expression studies in mammalian cells further support the consensus view that IP 3 Ks negatively regulate Ins(1,4,5)P 3 Ca 2ϩ signals by their ability to remove Ins(1,4,5)P 3 (19 -25). The product of these enzymes, Ins(1,3,4,5)P 4 , may further regulate Ca 2ϩ homeostasis, although exactly how it may do that remains controversial (10).From the first studies of the IP 3 Ks, it has been assumed that because they are enzymes with a higher affinity for Ins(1,4,5)P 3 than the Type I Ins(1,4,5)P 3 5-phosphatase (26) and differ from the 5-phosphatase in that they are positively regulated by CaM and CaM kinase (see Refs. 13,14,27, and 28 for reviews), they must play a preferential role in regulating the pools of Ins (1,4,5)P 3 involved in signaling. However, the exact nature of this contribution is st...

IP3K (inositol 1,4,5-trisphosphate 3-kinase) catalyses the Ca2+-regulated phosphorylation of the second messenger Ins(1,4,5)P3, thereby inactivating the signal to release Ca2+ and generating Ins(1,3,4,5)P4. Here we have investigated the localization and activity of IP3KB and its modulation by proteolysis. We found that the N- and C-termini (either side of residue 262) of IP3KB localized predominantly to the actin cytoskeleton and ER (endoplasmic reticulum) respectively, both in COS-7 cells and in primary astrocytes. The functional relevance of this was demonstrated by showing that full-length (actin-localized) IP3KB abolished the histamine-induced Ca2+ response in HeLa cells more effectively than truncated constructs localized to the ER or cytosol. The superior efficacy of full-length IP3KB was also attenuated by disruption of the actin cytoskeleton. By transfecting COS-7 cells with double-tagged IP3KB, we show that the translocation from actin to ER may be a physiologically regulated process caused by Ca2+-modulated constitutive proteolysis in intact cells.