Roles of Cross-Membrane Transport and Signaling in the Maintenance of Cellular Homeostasis

Organelles allow specialized functions within cells to be localized, contained and independently regulated. This separation is oftentimes achieved by selectively permeable membranes, which enable control of molecular transport, signaling between compartments and containment of stress-inducing factors. Here we consider the role of a number of membrane systems within the cell: the plasma membrane, that of the endoplasmic reticulum, and then focusing on the nucleus, depository for chromatin and regulatory centre of the cell. Nuclear pores allow shuttling of ions, metabolites, proteins and mRNA to and from the nucleus. The activity of transcription factors and signaling molecules is also modulated by translocation across the nuclear envelope. Many of these processes require 'active transportation' against a concentration gradient and may be regulated by the nuclear pores, Ran-GTP activity and the nuclear lamina. Cells must respond to a combination of biochemical and physical inputs and we discuss too how mechanical signals are carried from outside the cell into the nucleus through integrins, the cytoskeleton and the 'linker of nucleo- and cyto-skeletal' (LINC) complex which spans the nuclear envelope. Regulation and response to signals and stresses, both internal and external, allow cells to maintain homeostasis within functional tissue.