Multiple forms of p55PIK, a regulatory subunit of phosphoinositide 3-kinase, are generated by alternative initiation of translation.
A cDNA encoding p55PIK, one of the regulatory subunits of phosphoinositide (phosphatidylinositol) 3-kinase, was cloned from a cDNA library derived from the mouse mammary epithelial cell line C57MG. The cDNA coding for full-length p55PIK was transiently expressed in COS-7 cells. Western blot analysis of p55PIK expression using a specific antibody against p55PIK revealed that multiple protein products with different molecular masses were detected in COS-7 cell extracts. Experiments presented here demonstrate that multiple forms of p55PIK detected in COS-7 cells were produced by alternative initiation of translation. We also show that at least two in-frame start codons (AUG#2 and AUG#5) in p55PIK mRNA are used in COS-7 cells for the initiation of translation of p55PIK into proteins of 54 kDa and 50 kDa respectively. p55PIK mRNA was also alternatively translated into two proteins in PC cells, a mouse teratoma cell line, indicating that the alternative initiation of translation of p55PIK is not restricted to COS-7 cells. Results from immunoprecipitation and Western blot analysis showed that two forms (54 kDa and 50 kDa protein species) of p55PIK were detected in C57MG cells. Interestingly, when C57MG cells were treated with insulin, only p55PIK, but not p50PIK, bound to insulin receptor substrate-1 protein, providing evidence that different forms of p55PIKs may have specific distinct roles in signal transduction pathways.
Xia
X
X
Department of Pharmaceutical Sciences, University of Maryland School of Pharmacy, 20 N. Pine St., Baltimore, MD 21201, USA.
Serrero
G
G
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D019556:COS Cells; D003001:Cloning, Molecular; D018076:DNA, Complementary; D007328:Insulin / Q000494:pharmacology; D055504:Insulin Receptor Substrate Proteins; D047908:Intracellular Signaling Peptides and Proteins; D007527:Isoenzymes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D016366:Open Reading Frames; D019869:Phosphatidylinositol 3-Kinases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D010750:Phosphoproteins / Q000378:metabolism; D011485:Protein Binding; D014176:Protein Biosynthesis; D015398:Signal Transduction
D018076:DNA, Complementary; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D047908:Intracellular Signaling Peptides and Proteins; C526295:Irs1 protein, mouse; C526298:Irs2 protein, mouse; D007527:Isoenzymes; D010750:Phosphoproteins; D019869:Phosphatidylinositol 3-Kinases