Mammary gland casein kinases

If X is basic or particularly bulky, it may reduce the extent of phosphorylation the best example is provided by the as2-casein C variant in which lie replaces Thr-130 to reduce the extent of phosphorylation of Ser-129 and Ser-131. Although the triplet recognition site is necessary for phosphorylation by mammary Golgi kinases, not all sites are actually phosphorylated, so there may be a further topological requirement, such as surface location for the site or even a p-turn conformation. Some evidence for this has come from studies on the kinetics of phosphorylation of synthetic model substrates for mammary gland and other casein kinases (Meggio et al., 1989). 

Casein kinases exist as two distinct sets of enzymes [88]. The casein kinase in mammary tissue that normally provides in vivo phosphorylation of nascent proteins is distinct from the multisubstrate and ubiquitous casein kinase (CK-2) that is responsible for in vitro casein phosphorylation. The substrate specificity of the two enzymes is also different. For the casein kinase from mammary gland, the recognition sequence corresponds to the tripeptide Ser/Thr-X-Glu/Ser-P or Ser/ Thr-X-Glu/Ser in nonphosphorylated proteins, where X is any amino acid [89]. For CK-2, the recognition sites have been identified as Ser-Glu-Ala-Glu-Glu-Glu and Ser-Ala-Ala-Glu-Glu-Glu [88]. 

Table 4 Phosphorylation of Proteins by a Casein Kinase from Rat Mammary Gland...

Besides its in vivo activity, mammary gland casein kinase can phosphorylate proteins other than casein, as demonstrated by the in vitro phosphorylation of several food proteins as shown in Table 4 [90], As expected, there is a marked preference of the enzyme for dephosphorylated caseins over native casein, and for native caseins over other food proteins. So far, no protein kinases have been evaluated for the phosphorylation of plant proteins. An examination of known primary structural sequences of soy glycinin and /3-conglyci-nin revealed several potential phosphorylation sites for the mammary gland kinase but not for the CK-2 (unpublished observations). Based on this information, mammary gland kinase appears most likely to be active toward soy proteins. 

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