Abstract
Intracellar distribution of selected reticuloplasmin, soluble proteins of the endoplasmic reticulum lumen, in rat mammary gland was investigated during pregnancy, lactation and involution. During lactation the levels of the calcium binding protein calreticulin, and of protein disulfied isomerase, were elevated. Endoplasmic reticulum was as efficient as Golgi apparatus in sequestration and accumulation of Ca 2+ from surrounding meduim, as suggested from in vitro experiments with isolated cell fractions. Both protein disulfide isomerase and calcerticulin were present in cytosol fron homogenates of mammary gland prepared under mild conditions. Protein disulfide isomerse was abundant in intracellar lipid droplet precursors of milk lipid globules. Calreticulin and immunoglobulin binding protein (BiP, GRP 78) were associated with lipid droplets. Glucose-regulated protein (GRP 94) was not detected in association with intracellular lipid droplets. Milk lipids globule membrane lacked more than barely detectable quantities of protein disulfied isomerased, calreticulin, and immunoglobulin binding protein, suggesting that these proteins are lost from intracellular lipid droplets before or during their secretion as milk globules. Immunocytochemical localization confirmed the presence of protein disulfide isomerase or calreticulin on inracellular lipid droplets and in non-endoplasmic reticulum regions of cells.
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