To understand why mammalian cells are capable of producing secreted proteins of high quality in a highly coordinated manner, we have developed a system to examine single molecular dynamics of cargo proteins in the endoplasmic reticulum. Series of analyses have indicated that lumen of the ER are equipped with a machinery that restricts simple diffusion of cargo proteins through a grip of N-linked oligosaccharides using actin cytoskeleton. Since this regulation becomes most prominent at higher temperature, we propose that this constitutes a basic machinery for cargo maturation.
