Molecule of the Month: Golgi Casein Kinase

Many early biomolecular discoveries were made using proteins that are easy to obtain and purify. For example, the first structures of proteins were determined using myoglobin from a particularly plentiful source (whale muscle), and hemoglobin, which is readily purified from blood. In the late 1800s, scientists discovered that casein, the most plentiful protein in milk, contains phosphorus. Subsequent research revealed that this phosphorus is part of phosphate groups attached to serine amino acids in the protein. Since then, many additional phosphoproteins have been discovered, along with a large collection of kinases that add phosphate groups to proteins and phosphatases that take them off.

If you search the PDB archive for “casein kinase,” you’ll get a list of many, many structures. The name of these kinases, however, is a bit of a misnomer. They’re called “casein kinases” because they were discovered through their ability to phosphorylate casein. However, for most of them, that’s not their major biological function. Rather, they play essential roles in signalling in the cell cytoplasm, so they typically never come into contact with casein as it is built and secreted by the Golgi. The actual kinase that phosphorylates milk casein was only discovered in 2012.

The Golgi casein kinase, shown here from PDB entry 5yh2, adds phosphates to casein and also to many other types of secreted proteins. It is most active as a complex of two similar types of proteins. Fam20C is the catalytic subunit. It binds to casein and transfers a phosphate from ATP to the protein. Fam20A is not catalytically active, but it binds to Fam20C and makes it more active. For this reason, it is often called a "pseudokinase" because it's structurally similar to other kinases but isn't an enzyme. In addition, a third protein, Fam20B, is similar to these two, but adds phosphates to sugars.

Milk is a complex mixture of proteins, fats, and nutrients that provides everything that a growing infant needs. Most of the protein in cow’s milk is casein, whereas human milk has lesser amounts of casein. Casein chains are largely unstructured (so you won’t currently find any entries for casein the PDB archive) and are associated into large “micelles” in milk. Casein chains have regions that are highly phosphorylated, which associate with tiny mineral nanoclusters of calcium and phosphate in the micelle. The remaining portions of the casein chains are hydrophobic, and stick to each other. A specialized casein, called kappa-casein, coats the surface of the micelle. The outer portions of kappa-casein are negatively charged and glycosylated, which helps to make the whole micelle soluble.

A few familiar properties of milk are due to the micellar structure of casein. The large micelles, along with the large fat globules, scatter light and consequently make milk opaque and white. Also, the process of making cheese relies on the structure of the micelle. Milk is treated with rennet, which has enzymes that clip off the extended regions of kappa-casein. The trimmed micelles are not nearly as soluble and aggregate into curds. The surrounding milk proteins then make up the liquid whey.