Molecular biology

Intro

Kappa casein is the protein responsible for milk coagulation into cheese when it is cleaved by rennet (chymosin). If we could express this protein in yeast, we could essentially make vegan cheese.

A 2005 paper showed that the C-terminal part of the human kappa casein protein can be overexpressed in S. cerevisiae (baker’s yeast) and Pichia pastoris, and they were able to get it secreted in sufficient quantity that they could run the culture medium on a gel, and see a clear band for the protein. (“This macropeptide has various biological activities and is used as a functional food ingredient as well as a pharmaceutical compound.“ - valuable stuff, not just for vegan cheese!)

Our first task would be to replicate this result - ideally in the BioBricks format. Next would be to express and secrete the full-length protein. If we can get it to express and secrete in sufficient quantity, we should be able to show that you get a single band on the gel for the intact protein, but two bands after cleaving the protein with chymosin.

We can have some people working simultaneously on trying to express the other casein proteins (alpha-s1, alpha-s2, and beta). But since these are hydrophobic, we may not be able to express and secrete them without kappa-casein to hold them in suspension in a casein micelle. We can also attempt to modify the yeast lipid biosynthesis pathways to produce milk fats.

Essential reading

• Rennet coagulation of milk
• Mechanisms of Coagulation: The principles, the science and what they mean to cheesemakers
• Production of human caseinomacropeptide in recombinant Saccharomyces cerevisiae and Pichia pastoris (on seafile)

Caseinomacropeptide

Kim YJ, Oh YK, Kang W, Lee EY, Park S. Production of human caseinomacropeptide in recombinant Saccharomyces cerevisiae and Pichia pastoris. J Ind Microbiol Biotechnol. 2005 Sep;32(9):402-8.

“Caseinomacropeptide is a polypeptide of 64 amino acid residues (106-169) derived from the C-terminal part of the mammalian milk k-casein. This macropeptide has various biological activities and is used as a functional food ingredient as well as a pharmaceutical compound. The gene encoding the human caseinomacropeptide (hCMP) was synthesized and expressed with an alpha-factor secretion signal in the two yeast strains, Saccharomyces cerevisiae and Pichia pastoris. The complete polypeptide of the recombinant hCMP was produced and secreted in a culture medium by both the strains, but the highest production was observed in S. cerevisiae with a galactose-inducible promoter. In a fed-batch bioreactor culture, 2.5 g/l of the recombinant hCMP was obtained from the S. cerevisiae at 97 h.” “Escherichia coli XL1-Blue (Stratagene, USA) was used for cloning and propagating genes. Host strains for recombinant hCMP were S. cerevisiae 2805 (his-, ura-) [...] For S. cerevisiae, pYIGP (containing a constitutive GAP promoter) or pYEGa (containing a galactose-inducible GAL promoter) was used.”

Yeast expression

Strain selection

The Johns Hopkins' team used YPH500

• List of commonly used lab strains
• A comparison of four strains (article access?)

Vectors

The Jons Hopkins iGEM team modified pRS400-series vectors, removing the MCS and replacing them with biobrick prefix and suffix.

Patrick says that galactose induction is the most common form of controlling heterologous expression. Common vectors are the pYES2 and pESC series vectors.

Secretion

Secretion of proteins will require a secretion signal (triggering processing through the ER, golgi and then exocytosis).

iGem Parts

• VitaYeast project, by Johns Hopkins’ 2011 iGEM team
  • Characterized promoters and terminators
• Yeast parts in the BioBricks parts registry

Other notes

pESC contains a bidirectional promoter which may allow expression of both parts of the protein without cleavage being required. How?

Casein

Patrik D says: In a bout of procrastination from grant writing, I actually stumbled upon some more really good references on casein proteins and their recombinant expression. Turns out they're not quite as insoluble as I had feared, especially if you keep the Ca concentration low enough. They've even been explored as chaperones to get other problematic proteins to fold better. There's also some papers on alpha- and beta-casein expression in E. coli and yeast, and on reconstitution of casein micelles from beta and kappa casein (beta casein is the major component in human milk).

Kappa-casein

• Kappa-casein on wikipedia
• Bovine kappa-casein precursor gene CSN3
• kappa-casein genes in other mammals

Sequence

Bovine kappa-casein is 190 AA of which the first 21 AA form a secretion signal:

MMKSFFLVVT ILALTLPFLG AQEQNQEQPI RCEKDERFFS DKIAKYIPIQ YVLSRYPSYG 
LNYYQQKPVA LINNQFLPYP YYAKPAAVRS PAQILQWQVL SNTVPAKSCQ AQPTTMARHP 
HPHLSFMAIP PKKNQDKTEI PTINTIASGE PTSTPTTEAV ESTVATLEDS PEVIESPPEI 
NTVQVTSTAV 

Post-translational modification

According to the Uniprot record, the post-translation modifications include:

• Glycosylation of six residues, all of them Threonine ((O-GalNAc
• Phosphorylation of three residues, all of them Serine
• One disulphide bond between two cysteine residues
• One "Pyrrolidone carboxylic acid" modification to a glutamine (?)

We should investigate if any of this is important for micelle formation or micellar flocculation.

Clotting / Micellar flocculation

• Kinetics of milk coagulation: II. Kinetics of the secondary phase: micelle flocculation. (Carlson et al, 1987)
• Casein Interactions: Casting Light on the Black Boxes, the Structure in Dairy Products (Home, 1998, review)
• Studies of casein micelle structure: the past and the present (Qi, 2007, review)

If the model proposed by Home is correct, then micelle formation will require alpha, beta, kappa-casein and colloidal calcium. The model also implies that the phosphoserine residues of both alpha and kappa-casein play an important role in micelle formation.

Synthetic casein micelles

Some researchers have created synthetic casein micelles. We should study their efforts:

• Sub-structure of synthetic casein micelles (Knoop et al, 1979)
• A phosphate-induced sub-micelle-micelle equilibrium in reconstituted casein micelle systems (Slattery, 1979)