Golgi Casein Kinase

Casein and many other secreted proteins are phosphorylated by Golgi casein kinase

Golgi casein kinase, with Fam20C in blue, Fam20A in green, and ATP in magenta. A dimer of the two proteins is shown here. The proteins further associate into a tetramer in the crystal, but the significance of this is still under study.
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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 muscles), 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.

Kinases Everywhere

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’s built and secreted by the Golgi. The actual kinase that phosphorylates the casein in milk was only discovered in 2012.

Authentic Casein Kinase

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 binds to Fam20C and makes it more active. In addition, a third protein, Fam20B, is similar to these two, but adds phosphates to sugars.

This illustration shows a cross section through a casein micelle (tan, lower center) and a fat globule (yellow, upper left). The micelle includes many unstructured alpha and beta casein chains interacting with small calcium phosphate nanoclusters (white), and kappa casein chains extending from the surface. The fat globule is surrounded by a multi-layered membrane with many embedded proteins, filled with fat molecules (yellow) and a few carotene molecules (orange). Whey proteins are shown in darker shades around the micelle.
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Casein in Milk

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.

Milk and Cheese

A few familiar properties of milk are due to the micellar structure of casein. The size of the micelle, along with the large fat globules, scatters light and makes 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 whey.

Exploring the Structure

Phosphorylated Serine

Golgi casein kinase tends to add phosphate groups at locations in proteins with a characteristic sequence motif: S-X-E, where “S” is the serine to which the phosphate will be added, “X” can be anything, and “E” is a glutamate. The enzyme also works well if a previously phosphorylated serine is in the “E” position. PDB ID 2lid includes an example of this motif with a phosphate added. The protein is vitellogenin, a highly phosphorylated protein that is found in egg yolks. The example shown here is a similar vitellogenin from a wasp. To explore this structure in more detail, click on the image for an interactive JSmol.

Topics for Further Discussion

  1. You can compare the structures of Fam20C (chain D in 5yh2) and Fam20A (chain A in 5yh2) using the Pairwise Structure Alignment Tool. Also try aligning Fam20B (chain A in 5xoo).
  2. The PDB includes many structures of proteins with phosphorylated serine amino acids. To find them, look at the page for phosphoserine (SEP), and click on "is present in a polymer sequence".

References

  1. Worby, C.A., Mayfield, J.E., Pollack, A.J., Dixon, J.E., Bannerjee, S. (2021) The ABCs of the atypical Fam20 secretory pathway kinases. J Biol Chem 296: 100267
  2. Roy, D., Ye, A., Moughan, P.J., Singh, H. (2020) Composition, structure, and digestive dynamics of milk from different species. Front. Nutrition 7: 577759
  3. 5yh2: Zhang, H., Zhu, Q., Cui, J., Wang, Y., Chen, M.J., Guo, X., Tagliabracci, V.S., Dixon, J.E., Xiao, J. (2018) Structure and evolution of the Fam20 kinases. Nat Commun 9: 1218-1218
  4. 2lid: Havukainen, H., Underhaug, J., Wolschin, F., Amdam, G., Halskau, O. (2012) A vitellogenin polyserine cleavage site: highly disordered conformation protected from proteolysis by phosphorylation. J Exp Biol 215: 1837-184

January 2022, David Goodsell

doi:10.2210/rcsb_pdb/mom_2022_1
About Molecule of the Month
The RCSB PDB Molecule of the Month by David S. Goodsell (The Scripps Research Institute and the RCSB PDB) presents short accounts on selected molecules from the Protein Data Bank. Each installment includes an introduction to the structure and function of the molecule, a discussion of the relevance of the molecule to human health and welfare, and suggestions for how visitors might view these structures and access further details. More
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