Huang X, Honkanen R E
Department of Biochemistry and Molecular Biology, College of Medicine, University of South Alabama, Mobile 36688, USA.
J Biol Chem. 1998 Jan 16;273(3):1462-8. doi: 10.1074/jbc.273.3.1462.
A novel serine/threonine protein phosphatase (PPase) designated PP7 was identified from cDNA produced from human retina RNA. PP7 has a molecular mass of approximately 75 kDa, and the deduced amino acid sequence of PP7 contains a phosphatase catalytic core domain that possesses all of the invariant motifs of the PP1, PP2A, PP2B, PP4, PP5, and PP6 gene family. However, PP7 has unique N- and C-terminal regions and shares < 35% identity with the other known PPases. The unique C-terminal region of PP7 contains multiple Ca2+ binding sites (i.e. EF-hand motifs). This region of PP7 is similar to the Drosophila retinal degeneration C gene product (rdgC), and PP7 and rdgC share 42.1% identity. Unlike the other known PPases, the expression of PP7 is not ubiquitous; PP7 was only detected in retina and retinal-derived Y-79 retinoblastoma cells. Expression of recombinant human PP7 in baculovirus-infected SF21 insect cells produces an active soluble enzyme that is capable of utilizing phosphohistone and p-nitrophenyl phosphate as substrates. The activity of recombinant PP7 is dependent on Mg2+ and is activated by calcium (IC50 approximately equal to 250 microM). PP7 is not affected by calmodulin and is insensitive to inhibition by okadaic acid, microcystin-LR, calyculin A, and cantharidin.
从人视网膜RNA产生的cDNA中鉴定出一种名为PP7的新型丝氨酸/苏氨酸蛋白磷酸酶（PPase）。PP7的分子量约为75 kDa，推导的PP7氨基酸序列包含一个磷酸酶催化核心结构域，该结构域具有PP1、PP2A、PP2B、PP4、PP5和PP6基因家族的所有保守基序。然而，PP7具有独特的N端和C端区域，与其他已知的PPase的同源性小于35%。PP7独特的C端区域包含多个Ca2+结合位点（即EF手基序）。PP7的这一区域与果蝇视网膜变性C基因产物（rdgC）相似，PP7与rdgC的同源性为42.1%。与其他已知的PPase不同，PP7的表达并非普遍存在；仅在视网膜和视网膜来源的Y-79视网膜母细胞瘤细胞中检测到PP7。在杆状病毒感染的SF21昆虫细胞中重组人PP7的表达产生一种活性可溶性酶，该酶能够以磷酸组蛋白和对硝基苯磷酸为底物。重组PP7的活性依赖于Mg2+，并被钙激活（IC50约等于250 microM）。PP7不受钙调蛋白影响，对冈田酸、微囊藻毒素-LR、花萼海绵诱癌素A和斑蝥素的抑制不敏感。
