title: Evolution of major milk proteins in Mus musculus and Mus spretus mouse species: a genoproteomic analysis.
abstract: BACKGROUND
  Due to their high level of genotypic and phenotypic variability, Mus spretus strains were introduced in laboratories to investigate the genetic determinism of complex phenotypes including quantitative trait loci. Mus spretus diverged from Mus musculus around 2.5 million years ago and exhibits on average a single nucleotide polymorphism (SNP) in every 100 base pairs when compared with any of the classical laboratory strains. A genoproteomic approach was used to assess polymorphism of the major milk proteins between SEG/Pas and C57BL/6J, two inbred strains of mice representative of Mus spretus and Mus musculus species, respectively.
  
  
  RESULTS
  The milk protein concentration was dramatically reduced in the SEG/Pas strain by comparison with the C57BL/6J strain (34 ± 9 g/L vs. 125 ± 12 g/L, respectively). Nine major proteins were identified in both milks using RP-HPLC, bi-dimensional electrophoresis and MALDI-Tof mass spectrometry. Two caseins (β and αs1) and the whey acidic protein (WAP), showed distinct chromatographic and electrophoresis behaviours. These differences were partly explained by the occurrence of amino acid substitutions and splicing variants revealed by cDNA sequencing. A total of 34 SNPs were identified in the coding and 3'untranslated regions of the SEG/Pas Csn1s1 (11), Csn2 (7) and Wap (8) genes. In addition, a 3 nucleotide deletion leading to the loss of a serine residue at position 93 was found in the SEG/Pas Wap gene.
  
  
  CONCLUSIONS
  SNP frequencies found in three milk protein-encoding genes between Mus spretus and Mus musculus is twice the values previously reported at the whole genome level. However, the protein structure and post-translational modifications seem not to be affected by SNPs characterized in our study. Splicing mechanisms (cryptic splice site usage, exon skipping, error-prone junction sequence), already identified in casein genes from other species, likely explain the existence of multiple αs1-casein isoforms both in SEG/Pas and C57BL/6J strains. Finally, we propose a possible mechanism by which the hallmark tandem duplication of a 18-nt exon (14 copies) may have occurred in the mouse genome.
authors.affiliation: INRA, UR1313 Génétique animale et Biologie intégrative UMR 1313, Equipe LGS, Jouy-en-Josas, France.
authors.forename: Nisrine
authors.initials: N
authors.lastname: Boumahrou
authors.forename: Claudia
authors.initials: C
authors.lastname: Bevilacqua
authors.forename: Christian
authors.initials: C
authors.lastname: Beauvallet
authors.forename: Guy
authors.initials: G
authors.lastname: Miranda
authors.forename: Sanda
authors.initials: S
authors.lastname: Andrei
authors.forename: Emmanuelle
authors.initials: E
authors.lastname: Rebours
authors.forename: Jean-Jacques
authors.initials: JJ
authors.lastname: Panthier
authors.forename: Sylvain
authors.initials: S
authors.lastname: Bellier
authors.forename: Patrice
authors.initials: P
authors.lastname: Martin
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002851:Chromatography, High Pressure Liquid; D015180:Electrophoresis, Gel, Two-Dimensional; D004591:Electrophoresis, Polyacrylamide Gel; D019143:Evolution, Molecular; D005260:Female; D023281:Genomics; D051379:Mice / Q000235:genetics*; D008810:Mice, Inbred C57BL; D008894:Milk Proteins / Q000235:genetics*; D008969:Molecular Sequence Data; D020641:Polymorphism, Single Nucleotide; D011499:Protein Processing, Post-Translational; D019154:Protein Splicing; D040901:Proteomics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017422:Sequence Analysis, DNA; D017384:Sequence Deletion; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
chemical_list: D008894:Milk Proteins