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¹ Division of Clinical Immunology,
² Department of Genetics and Pathology,
³ Division of Biomedical Radiation Sciences, Rudbeck Laboratory, Uppsala University, Uppsala, Sweden
⁴ Ludwig Institute for Cancer Research, Uppsala, Sweden
⁵ Department of Endocrine Oncology, University Hospital, Uppsala, Sweden

(Requests for offprints should be addressed to V Giandomenico; Email: valeria.giandomenico{at}klinimm.uu.se)

Vesicular monoamine transporter 1 (VMAT1) is an integral protein in the membrane of secretory vesicles of neuroendocrine and endocrine cells that allows the transport of biogenic monoamines, such as serotonin, from the cytoplasm into the secretory vesicles. The full-length VMAT1 transcript is produced from 16 exons. We have identified and characterized an alternatively spliced form of VMAT1 that lacks exon 15, the next to last exon of VMAT1. The new form was therefore denoted VMAT115. Exon 15 does not contain an even multiple of three nucleotides. As a consequence, there is a shift of reading frame, and exon 16 is translated in an alternative reading frame, yielding a novel protein with a shorter and unrelated C-terminus compared with the native VMAT1 protein. VMAT1 and VMAT115 mRNAs are simultaneously expressed in normal and neoplastic neuroendocrine cells of the GI tract. However, VMAT1 expression is always higher than VMAT115 expression. We prove that VMAT115 is not localized in large, dense core vesicles as the native form but in the endoplasmic reticulum. Furthermore, while VMAT1 can take up serotonin, VMAT115 cannot, indicating different functions for the two forms of VMAT1.

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