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  • Title: Structural analysis of the mouse prosaposin (SGP-1) gene reveals the presence of an exon that is alternatively spliced in transcribed mRNAs.
    Author: Zhao Q, Hay N, Morales CR.
    Journal: Mol Reprod Dev; 1997 Sep; 48(1):1-8. PubMed ID: 9266755.
    Abstract:
    SGP-1/prosaposin can be secreted or targeted to the lysosomes where it is processed into smaller saposins A, B, C, and D required for the hydrolysis of glycosphingolipids. The deficiency of saposins B and C results in variant forms of metachromatic leukodystrophy and Gaucher's disease, respectively, which are characterized by lysosomal storage of undegraded glycosphingolipids. A required step to correct these genetic defects, or to understand the targeting mechanism of SGP-1 to the lysosomes, or to the extracellular space as well as its interaction with specific glycosphingolipids, is the analysis of the gene encoding this protein. Thus our investigation dealt with the molecular cloning of the mouse SGP-1 gene. Sequence analysis revealed that the mouse SGP-1 gene consists of 15 exons ranging from nine base pairs to 298 base pairs and 14 introns, which ranged from 89 base pairs to >8 kb in length. Our data show that saposin A is encoded by the exons 3, 4, and 5, saposin B by exons 6, 7, 8, and 9, saposin C by exons 10 and 11, and saposin D by exons 12, 13, and 14. The translation start codon is located within exon 1, and the translation stop codon is located within exon 15. The exon/intron boundaries were in accordance to the AG/GT consensus sequences. Our data also revealed that the SGP-1 gene has an exon consisting of the nine base pairs (CAG GAT CAG) encoding the three amino acids of saposin B, which may be alternatively spliced in the SGP-1 mRNA. The presence of the different forms of alternatively spliced mRNAs in various tissues was analyzed by RT-PCR. This approach demonstrated that prosaposin mRNAs of brain, heart, and muscle contain the nine base pairs of exon 8, whereas the transcripts from testis, lung, pancreas, spleen, and kidney do not contain this exon 8. Sequence comparison between the human and mouse prosaposin showed that exon 11 of mouse SGP-1 consists of 279 base pairs, whereas the human prosaposin gene consists of 187 base pairs. The extra 93 base pairs encode 31 amino acids corresponding to a proline-rich region located between saposin C and saposin D in the mouse prosaposin molecule. Finally, the availability of these genomic clones provides a starting point for further studies on the genetic role of specific sequences on the structure and function of SGP-1/prosaposin and its derived saposin proteins. In conclusion, we cloned and sequenced the mouse prosaposin (SGP-1) gene. The structural analysis of this gene revealed the presence of an exon that is alternatively spliced in transcribed mRNAs in a tissue-specific manner.
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