Abstract
In a previous study on camel milk from Kazakhstan, we reported the occurrence of two unknown proteins (UP1 and UP2) with different levels of phosphorylation. Here we show that UP1 and UP2 are isoforms of camel αs2-CN (αs2-CNsv1 and αs2-CNsv2, respectively) arising from alternative splicing events. First described as a 178 amino-acids long protein carrying eight phosphate groups, the major camel αs2-CN isoform (called here αs2-CN) has a molecular mass of 21,906 Da. αs2-CNsv1, a rather frequent (35%) isoform displaying a higher molecular mass (+1,033 Da), is present at four phosphorylation levels (8P to 11P). Using cDNA-sequencing, αs2-CNsv1 was shown to be a variant arising from the splicing-in of an in-frame 27-nucleotide sequence encoding the nonapeptide ENSKKTVDM, for which the presence at the genome level was confirmed. αs2-CNsv2, which appeared to be present at 8P to 12P, was shown to include an additional decapeptide (VKAYQIIPNL) revealed by LC-MS/MS, encoded by a 3′-extension of exon 16. Since milk proteins represent a reservoir of biologically active peptides, the molecular diversity generated by differential splicing might increase its content. To evaluate this possibility, we searched for bioactive peptides encrypted in the different camel αs2-CN isoforms, using an in silico approach. Several peptides, putatively released from the C-terminal part of camel αs2-CN isoforms after in silico digestion by proteases from the digestive tract, were predicted to display anti-bacterial and antihypertensive activities.
Highlights
Combining different proteomic approaches, the complexity of camel milk proteins was resolved to provide a detailed characterization of fifty protein molecules belonging to the 9 main milk protein families, including caseins: κ, αs2, αs1- and β-CN and two unknown proteins (UP1 and UP2), exhibiting molecular masses around 23,000 Da1
Whereas there is a substantial literature on bioactive peptides derived from bovine milk proteins[13] and more or less comprehensive databases of milk bioactive peptides exist[17,18,19,20], studies aiming at identifying peptides derived from camel milk proteins having potential health-promoting activities are scarce
The presence of two unknown proteins UP1 and UP2 with different phosphorylation levels was reported in our previous study[8]
Summary
CSN1S2 increasing its ability to Received: 9 May 2018 Accepted: 14 March 2019 Published: xx xx xxxx generate potentially bioactive peptides. Since milk proteins represent a reservoir of biologically active peptides, the molecular diversity generated by differential splicing might increase its content. To evaluate this possibility, we searched for bioactive peptides encrypted in the different camel αs2-CN isoforms, using an in silico approach. To substantiate the hypothesis according to which UP1 and UP2 might originate in CN and more precisely in αs-CN, we undertook characterizing more precisely these proteins In addition to their nutritional value, an increasing number of therapeutic effects and a variety of potential activities[11,12] are attributed to milk proteins as well as to milk-derived bioactive peptides encrypted in milk protein sequences[13]. Using an in silico approach, we searched for potential biological activities of sequences generated from alternative splicing of primary transcript encoding αs-CN
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