title: Identification of a Novel Transcript and Regulatory Mechanism for Microsomal Triglyceride Transfer Protein.
abstract: Microsomal triglyceride transfer protein (MTP) is essential for the assembly of triglyceride-rich apolipoprotein B-containing lipoproteins. Previous studies in our laboratory identified a novel splice variant of MTP in mice that we named MTP-B. MTP-B has a unique first exon (1B) located 2.7 kB upstream of the first exon (1A) for canonical MTP (MTP-A). The two mature isoforms, though nearly identical in sequence and function, have different tissue expression patterns. In this study we report the identification of a second MTP splice variant (MTP-C), which contains both exons 1B and 1A. MTP-C is expressed in all the tissues we tested. In cells transfected with MTP-C, protein expression was less than 15% of that found when the cells were transfected with MTP-A or MTP-B. In silico analysis of the 5'-UTR of MTP-C revealed seven ATGs upstream of the start site for MTP-A, which is the only viable start site in frame with the main coding sequence. One of those ATGs was located in the 5'-UTR for MTP-A. We generated reporter constructs in which the 5'-UTRs of MTP-A or MTP-C were inserted between an SV40 promoter and the coding sequence of the luciferase gene and transfected these constructs into HEK 293 cells. Luciferase activity was significantly reduced by the MTP-C 5'-UTR, but not by the MTP-A 5'-UTR. We conclude that alternative splicing plays a key role in regulating MTP expression by introducing unique 5'-UTRs, which contain elements that alter translation efficiency, enabling the cell to optimize MTP levels and activity.
authors.affiliation: Department of Pathology, Microbiology and Immunology, Vanderbilt University School of Medicine, Nashville, Tennessee, United States of America.
authors.forename: Takashi
authors.initials: T
authors.lastname: Suzuki
authors.affiliation: Department of Pathology, Microbiology and Immunology, Vanderbilt University School of Medicine, Nashville, Tennessee, United States of America.
authors.forename: Judy J
authors.initials: JJ
authors.lastname: Brown
authors.affiliation: Department of Pathology, Microbiology and Immunology, Vanderbilt University School of Medicine, Nashville, Tennessee, United States of America.|Research Service, Veterans Affairs, Tennessee Valley Health Care System, Nashville, Tennessee, United States of America.
authors.forename: Larry L
authors.initials: LL
authors.lastname: Swift
mesh_terms: D017398:Alternative Splicing / Q000235:genetics; D000818:Animals; D016466:CHO Cells; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D003412:Cricetulus; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D057809:HEK293 Cells; D006801:Humans; D051379:Mice; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics; D011817:Rabbits; D020133:Reverse Transcriptase Polymerase Chain Reaction
chemical_list: D002352:Carrier Proteins; D020033:Protein Isoforms; D012333:RNA, Messenger; C106092:microsomal triglyceride transfer protein