Identification and Functional Characterization of an Alternative 5 ’ Exon of the Sodium Dependent Ascorbic Acid Transporter SLC 23 A 1

Since the initial description of the human SLC23A1 transcript, encoding a sodiumdependent ascorbic acid transporter, no alternative splice variant had been described. In this report we describe a novel human specific alternative first exon which is located 1078 nucleotides 5’ of the canonical first exon. The two first exons are not mutually exclusive since they splice together to create a novel SLC23A1 protein isoform, we name it isoform 1A, with a N-terminus that is elongated by 36 amino acid. The novel SLC23A1 isoform locates to the plasma membrane, but mediates only 7% of the ascorbic acid transport exhibited by the shorter isoform when expressed in Xenopus