Modulation of Xenopus oocyte-expressed phospholemman-induced ion currents by co-expression of protein kinases.
Phospholemman (PLM), the major sarcolemmal substrate for phosphorylation by cAMP-dependent kinase (PKA) protein kinase C (PKC) and NIMA kinase in muscle, induces hyperpolarization-activated anion currents in Xenopus oocytes, most probably by enhancing endogenous oocyte currents. PLM peptides from the cytoplasmic tail are phosphorylated by PKA at S68, by NIMA kinase at S63, and by PKC at both S63 and S68. We have confirmed the phosphorylation sites in the intact protein, and we have investigated the role of phosphorylation in the regulatory activity of PLM using oocyte expression experiments. We found: (1) the cytoplasmic domain is not essential for inducing currents in oocytes; (2) co-expression of PKA increased the amplitude of oocyte currents and the amount of PLM in the oocyte membrane largely, but not exclusively, through phosphorylation of S68; (3) co-expression of PKA had no effect on a PLM mutant in which all putative phosphorylation sites had been inactivated by serine to alanine mutation (SSST 62, 63, 68, 69 AAAA); (4) co-expression of PKC had no effect in this system; (5) co-expression of NIMA kinase increased current amplitude and membrane protein level, but did not require PLM phosphorylation. These findings point to a role for phosphorylation in the function of PLM.
Mounsey
J P
JP
Department of Internal Medicine (Cardiovascular Division), University of Virginia Health Sciences Center, Charlottesville, VA 22908, USA. pmounsey@virginia.edu
Lu
K P
KP
Patel
M K
MK
Chen
Z H
ZH
Horne
L T
LT
John
J E
JE
Means
A R
AR
Jones
L R
LR
Moorman
J R
JR
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D018797:Cell Cycle Proteins; D002462:Cell Membrane / Q000378:metabolism; D018118:Chloride Channels / Q000096:biosynthesis; D017868:Cyclic AMP-Dependent Protein Kinases / Q000096:biosynthesis; D015870:Gene Expression; D007473:Ion Channels / Q000096:biosynthesis*; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism*; D008969:Molecular Sequence Data; D000072380:NIMA-Related Kinase 1; D000072379:NIMA-Related Kinases; D009865:Oocytes / Q000378:metabolism*; D010750:Phosphoproteins / Q000737:chemistry / Q000378:metabolism*; D010766:Phosphorylation; D011493:Protein Kinase C / Q000096:biosynthesis; D011494:Protein Kinases / Q000096:biosynthesis*; D017346:Protein Serine-Threonine Kinases / Q000096:biosynthesis; D015854:Up-Regulation; D014981:Xenopus
D018797:Cell Cycle Proteins; D018118:Chloride Channels; D007473:Ion Channels; D008565:Membrane Proteins; D010750:Phosphoproteins; C069141:phospholemman; D011494:Protein Kinases; D000072380:NIMA-Related Kinase 1; D000072379:NIMA-Related Kinases; C442331:NIMA-related kinase 6; D017346:Protein Serine-Threonine Kinases; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C