The gene encoding αs1-casein is expressed in human mammary epithelial cells during lactation

Abstract

Reverse-phase high performance liquid chromatograhy followed by N-terminal microsequencing performed directly on each peak collected, has provided a comprehensive survey of six individual human milk protein fractions. Sequencing of tryptic peptides arising from a putative αs1-casein fraction identifies sequences showing some similarity with αs1-casein from other species Ordering of these tryptic peptides and finally deciphering of the human αs1-casein amino acid sequence was achieved after cloning and sequencing of the relevant cDNA, amplified by reverse transcription-polymerase chain reaction starting from rnRNA extracted from mammary epithelial cells harvested from breast milk. Shorter than its ruminant counterparts (170 vs 199 amino acid residues) the human αs1-casein displays very low similarities with αs1-caseins known so far, very likely owing to combinatory splicing processes, characteristic for each species, as well as to genomic rearrangements. It is elsewhere distinguishable by the presence of three cysteinyl residues. Multiple forms of αs1-casein messengers were identified from each individual mRNA sample studied, strongly suggesting, therefore, a differential splicing from a single primary transcript. Finally, we provide definite evidence for the existence of a functional αs1-Cas locus in the human genome, which is expressed in the mammary tissue during lactation.