Identification of a novel isoform of microsomal triglyceride transfer protein.
Microsomal triglyceride transfer protein (MTP) has been studied extensively, primarily because of its role in the assembly of very low density lipoproteins by the liver and chylomicrons by the intestine. Recent studies have suggested that MTP may also play key roles in other cellular processes. In this paper we report the identification of a novel splice variant of MTP in mice. This isoform, MTP-B, has a unique first exon located approximately 2.7 kilobases upstream of canonical MTP (MTP-A) exon 1. The alternative exon encodes 35 amino acids compared with 20 amino acids encoded by exon 1 of MTP-A. MTP-B represents approximately 90% of total MTP mRNA in mouse adipocytes and 3T3-L1 cells and <5% in mouse liver and intestine. Expression of the alternate isoform in mouse liver was confirmed by mass spectrometry. Co-transfection of COS cells with truncated forms of apoB and either MTP-A or MTP-B demonstrated that both isoforms are effective in the assembly and secretion of nascent apoB-containing lipoproteins. Confocal microscopy of 3T3-L1 cells transfected with enhanced green fluorescent protein or DsRed fusions of the two proteins revealed that MTP-A is localized to the endoplasmic reticulum, whereas MTP-B localizes primarily to the Golgi complex in these cells. We conclude that MTP-B functions similarly to MTP-A in lipoprotein assembly. However, in nonlipoprotein-secreting cells, such as the adipocyte, MTP-B may have different localization properties, perhaps reflecting a distinct role in lipid storage and mobilization.
Mohler
Peter J
PJ
Department of Internal Medicine, University of Iowa School of Medicine, Iowa City, Iowa 52242.
Zhu
Mei-Ying
MY
Department of Pathology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-2561.
Blade
Anna M
AM
Department of Pathology, Wake Forest University School of Medicine, Winston-Salem, North Carolina 27157-1040.
Ham
Amy-Joan L
AL
Department of Biochemistry, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-2561.
Shelness
Gregory S
GS
Department of Pathology, Wake Forest University School of Medicine, Winston-Salem, North Carolina 27157-1040.
Swift
Larry L
LL
Department of Pathology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-2561. Electronic address: larry.swift@vanderbilt.edu.
D041721:3T3-L1 Cells; D000595:Amino Acid Sequence; D000818:Animals; D019556:COS Cells; D002352:Carrier Proteins / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification* / Q000502:physiology; D002522:Chlorocebus aethiops; D013058:Mass Spectrometry; D051379:Mice; D008810:Mice, Inbred C57BL; D008969:Molecular Sequence Data; D020033:Protein Isoforms; D014162:Transfection
D002352:Carrier Proteins; D020033:Protein Isoforms; C106092:microsomal triglyceride transfer protein