ABSTRACT
This chapter reviews protein phosphorylation in one prolactin target tissue, the mammary gland. In relating the molecular actions of prolactin to protein phosphorylations one will realize that few, if any, prolactin-regulated protein kinases or phosphatases have yet been documented. The phosphorylation of proteins occurs as an enzymatically mediated posttranslational modification. Phosphate is covalently attached to the side group of serine, threonine, or tyrosine. Phosphorylation of proteins may be permanent, such as caseins, or may be transient, as the phosphorylation and dephosphorylation reactions of substrate enzymes such as glycogen synthase. Protein phosphorylation has been successfully studied in several different experimental systems. These include in vivo studies, whole cell homogenates or subcellular organelles, and isolated protein systems. Protein kinases have been classified by a number of properties, including their substrate specificities, biochemical characteristics, or regulatory mechanisms. Fatty acid synthesis occurs in the cytosolic fatty acid synthase complex.
