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Identification of a cDNA coding for a fifth form of myelin basic protein in mouse
Abstract:
The primary sequences of four molecular mass variants (14, 17, 18.5, and 21.5 kDa) of the mouse myelin basic protein (MBP) have recently been determined through analysis of cDNA clones of their mRNAs. The mRNAs coding for the four MBP variants are thought to arise by differential splicing of two exons (exons 2 and 6) from a single gene. In contrast, exons 2 and 5 may be spliced out in the posttranscriptional processing of the human MBP gene. To investigate the possibility that a third exon (exon 5) may also be differentially spliced out in the processing of the mouse MBP gene transcript, a mouse cDNA library was screened to search for cDNAs missing exon 5. A MBP cDNA was isolated whose coding region specified a fifth mouse MBP variant with a molecular mass of approximately equal to 17 kDa. The mass of this variant (17,257 Da) is so close to that of the other 17-kDa mouse MBP (17,224 Da) that the two would be indistinguishable on NaDodSO4/polyacrylamide gels. Analysis of the sequence of the cDNA clone indicates that excision of exons 2 and 5 of the mouse MBP gene would produce the mRNA encoding this newly described 17-kDa MBP, whereas excision of exon 6 would produce the mRNA for the other 17-kDa MBP variant. Thus, the "17-kDa" mouse MBP consists of at least two molecular forms with very similar molecular masses but markedly different primary sequences. Of five full-length or near full-length cDNAs representing 17-kDa MBPs, one was missing exons 2 and 5 and four were missing exon 6.
Insights
Researchers discovered a new 17-kDa mouse myelin basic protein (MBP) variant. This finding reveals that the 17-kDa mouse MBP comprises at least two distinct molecular forms with different primary sequences, arising from alternative splicing.
Area of Science:
- Neuroscience
- Molecular Biology
- Genetics
Background:
- Myelin basic protein (MBP) is crucial for central nervous system myelination.
- Four mouse MBP variants (14, 17, 18.5, 21.5 kDa) arise from differential splicing, primarily involving exons 2 and 6.
- Human MBP processing may involve splicing out of exons 2 and 5.
Purpose of the Study:
- To investigate alternative splicing in mouse MBP gene expression.
- To determine if exon 5 is differentially spliced in mouse MBP.
- To identify novel MBP variants through cDNA library screening.
Main Methods:
- Screening of a mouse cDNA library to identify transcripts lacking exon 5.
- Sequence analysis of isolated cDNA clones.
- Comparison of primary sequences and predicted molecular masses of MBP variants.
Main Results:
- A fifth mouse MBP variant (approx. 17 kDa) was identified, differing from known variants.
- This new variant results from the splicing out of exons 2 and 5.
- The previously identified 17-kDa MBP variant arises from the splicing out of exon 6.
- These two 17-kDa variants have similar masses but distinct primary sequences.
Conclusions:
- The 17-kDa mouse MBP population consists of at least two molecular forms.
- Alternative splicing of mouse MBP gene, including exon 5, generates significant molecular diversity.
- Understanding MBP splicing is key to comprehending myelin structure and function.