Ehring G R, Lagos N, Zampighi G A, Hall J E
Department of Physiology and Biophysics, University of California, Irvine 92717.
J Membr Biol. 1992 Feb;126(1):75-88. doi: 10.1007/BF00233462.
Major intrinsic polypeptide (MIP), a 28-kDa protein isolated from lens fiber cell membranes, forms large, nonselective channels when reconstituted into lipid bilayers. MIP channels are regulated by voltage, such that these channels close when the potential across the membrane is greater than 30 mV. We have investigated the modulation of the voltage-dependent closure of MIP channels by phosphorylation. In this report, we describe the isolation of two isomers of MIP from lens fiber cell membranes. These isomers differ by a single phosphate at a protein kinase A phosphorylation site. The phosphorylated isomer produces channels that close in response to applied voltages when reconstituted into bilayers. The nonphosphorylated isomer produces voltage-independent channels. Direct phosphorylation with protein kinase A converts voltage-independent channels to voltage-dependent channels in situ. Analyses of macroscopic and single-channel currents suggest that phosphorylation increases the voltage-dependent closure of MIP channels by increasing closed channel lifetimes and the rate of channel closure following the application of voltage.
主要内在多肽（MIP）是一种从晶状体纤维细胞膜中分离出的28 kDa蛋白质，当重组到脂质双层中时会形成大的、非选择性的通道。MIP通道受电压调节，当膜两侧的电位大于30 mV时，这些通道会关闭。我们研究了磷酸化对MIP通道电压依赖性关闭的调节作用。在本报告中，我们描述了从晶状体纤维细胞膜中分离出的两种MIP异构体。这些异构体在蛋白激酶A磷酸化位点上仅相差一个磷酸基团。磷酸化的异构体产生的通道在重组到双层膜中时会响应施加的电压而关闭。未磷酸化的异构体产生电压不依赖的通道。用蛋白激酶A直接磷酸化可将电压不依赖的通道原位转化为电压依赖的通道。对宏观电流和单通道电流的分析表明，磷酸化通过增加关闭通道的寿命和施加电压后通道关闭的速率来增加MIP通道的电压依赖性关闭。
