Expression of the peptidoglycan recognition protein, PGRP, in the lactating mammary gland

The peptidoglycan recognition protein, PGRP, known as an intracellular component of neutrophils, has been isolated from camel (Camelus dromedarius) milk by acid precipitation followed by heparin-sepharose affinity chromatography of the supernatant. The mean concentration in milk was about 120 mg/L. It decreased during lactation by 19% and increased in the event of severe mastitis by 45%. The protein bound to lactic acid bacteria and other gram-positive bacteria with an affinity similar to that reported for the human and murine orthologs, although the isoelectric point of the molecule was distinctly higher at pH 9.02. The N-terminus of mature camel PGRP was determined as NH2-ArgGluAspProPro-CO2H. Calculated and measured molecular masses were both 19.1 kDa, excluding the possibility of posttranslational modifcation or binding of cation ligands. The peptide probably builds a homotrimer at high concentration. The corresponding mRNA was isolated from lactating mammary gland tissue, and 5.3 kbp of the corresponding gene was sequenced. Similarities were found to the camel lactoferrin gene with regard to sites of expression and to the region 5' upstream to the gene.