206 DAIRY CHEMISTRY AND BIOCHEMISTRYvia secretory vesicles, to the apical membrane. Covalent modification must
therefore occur at some point(s) along this route. Such modifications may
be either co-translational (occurring when chain elongation is in progress)
or post-translational. Proteolytic cleavage of the signal peptide is co-trans-
lational and this seems to be the case also for N-glycosylation, in which
dolichol-linked oligosaccharides are enzymatically transferred to aspara-
ginyl residues of the chain when these are present in the sequence code,
Asn-X-Thr/Ser (where X is any amino acid except proline). The large
oligosaccharide component may be 'trimmed' as it traverses the secretory
pathway. Formation of disulphide bonds between adjacent sections of the
chain, or between adjacent chains (as in K-casein), may also be partly
co-translational.
By contrast, O-glycosylation and O-phosphorylation appear to be post-
translational events. Glycosylation of the principal milk-specific glyco-
protein, casein, is believed to be effected by membrane-bound glycosyltrans-
ferases (three such enzymes have been described) located in the Golgi
apparatus. O-Phosphorylation involves transfer of the y-phosphate of ATP
to serine (or, less frequently, threonine) residues, occurring in the sequence,
Ser/Thr-X-A (where X is any amino acid residue and A is an acidic residue,
such as aspartic or glutamic acid or a phosphorylated amino acid). Phos-
phorylation is effected by casein kinases which are located chiefly in the
Golgi membranes. In addition to the correct triplets, the local conformation
of the protein is also important for phosphorylation of Ser since not all
serines in caseins in the correct sequence are phosphorylated. Some serine
residues in p-lg occur in a Ser-X-A sequence but are not phosphorylated,
probably due to extensive folding of this protein.
The Golgi complex is also the locus of casein micelle formation. In
association with calcium, which is actively accumulated by Golgi vesicles,
the polypeptide chains associate to form submicelles, and then micelles,
prior to secretion.
4.14.5 Structure and expression of milk protein genes
The structure, organization and expression of milk protein genes are now
understood in considerable detail. This subject is considered to be outside
the scope of this book and the interested reader is referred to Mepham et
al. (1992). Such knowledge permits the genetic engineering of milk proteins
with respect to the transfer of genes from one species to another, the
overexpression of a particular desirable protein(s), the elimination of certain
undesirable proteins, changing the amino acid sequence by point mutations
to modify the functional properties of the protein or transfer of a milk
protein gene to a plant or microbial host. This topic is also considered to
be outside the scope of this text and the interested reader is referred to
Richardson et al. (1992).