Expression of fatty acid-binding proteins in the developing mouse mammary gland

Proteins of 14,000 daltons (SLP-14) were isolated and partially characterized from mouse mammary glands of different developmental stages. The purified proteins were partially sequenced at the amino acid level. The SLP-14 belong to the family of fatty acid-binding (FABP) proteins. The major SLP-14 expressed was a reflection of the differentiation stage of the mammary gland. Within the regions sequenced for each protein, virgin mouse mammary gland (primarily adipocytes) expressed a protein 100% homologous to adipocyte lipid binding protein whereas pregnant and lactating mouse mammary glands expressed a protein highly homologous to mouse heart fatty acid binding protein. In the lactating gland, the isolated protein was 97% identical to the heart FABP over a 98 amino acid stretch. We could not detect the 14Kd protein identified as mammary gland growth inhibitor (MDGI) which is also a member of the FABP family and highly homologous to heart FABP. A rabbit antiserum to the rat mammary SLP-14 recognized the SLP-14 proteins in mouse and rat mammary gland, skeletal muscle and heart, whereas it failed to recognize the SLP-14 in liver, intestine and other organs. These data indicate that the SLP-14 detected in rodent mammary gland is of the fatty acid-binding protein family. On the basis of amino acid sequence, the major form in the differentiated mouse mammary gland is apparently FABP and not MDGI. MDGI may be a protein in low abundance and/or localized to a specific group of mammary epithelial cells.