In eukaryotic cells, the cytoplasmic concentration of small ionic molecules (e.g., fatty acids, acyl-CoA and calcium) in a free state is normally kept at very low levels. Intracellular binding proteins often increase the total concentration of these molecules above their free level thus facilitating their transport and metabolism. Herein, we review work done by us and others on structural and functional aspects of three different types of small (10-15 kDa), abundantly expressed, cytoplasmic binding proteins: calgranulins, fatty acid- and acyl-CoA-binding proteins.

Calgranulins are calcium-binding proteins of the S100 protein family, primarily expressed in peripheral granulocytes. We have isolated and characterized pig calgranulin A, calgranulin B, and a novel, related protein named calgranulin C.

Over 15 fatty acid-binding proteins from animals belonging to different zoological classes were isolated and their partial or total primary structure determined in our laboratory. We review relevant data involving functional aspects of the various fatty acid- binding protein types, including their ligand binding specificity and interaction with membranes or enzymes.

Acyl-CoA-binding proteins bind medium- and long-chain acyl-CoA esters with high affinity and specificity. We recently reported for the first time the presence of both, an acyl-CoA-binding protein and a fatty acid-binding protein in the Harderian gland. The latter at a very low concentration in comparison with the presence of high amounts of the acyl-CoA-binding protein. The relevance of these proteins in the lipid secreting function of the Harderian gland is discussed.